docs: split C4 route 4 into 4a/4b and pin the 4a contract

User-directed after scoping put whole-route 4 at 1,500-2,500 production lines
against a stated ~400 budget.

4a is the steady state: the classifier's Interpolate (contact, < 96 m) and
NoPositionOperation (no contact) branches. Neither performs a SetPosition, so 4a
carries no deferred-cell park, no service-window guard, no allocation exposure,
and no interaction with the Forget-on-every-accepted-Position behaviour that
dominated route 2's review rounds. It also fixes two of the three unfiled
divergences: the NPC airborne hard-snap that ignores the wire IsGrounded bit
(retail returns 0 and writes nothing, MoveOrTeleport @0x0051636D), and
ConstrainTo armed before the operation instead of after (retail arms it post-move
only on a nonzero return, @0x00454272).

4b takes the edges — teleport, far-snap, cell-less — where the parks, the
Position-time service-window guard, #277's broken bound, N3, and the third
divergence live.

The contract sanctions exactly one dual path: 4a routes its two classifications
through the new seam and leaves the other two on the legacy path until 4b. That
is a staged cutover rather than a duplicate authority ONLY because the
discriminator is the classifier itself and the classifications are mutually
exclusive; the contract says so explicitly and requires the fallback deleted in
4b.

Two carried acdream additions are called out as load-bearing rather than left to
be discovered: AP-87's 4 m / !willBeDrTicked snap conditions (which prevent the
#184 invisible-but-solid monster and are NOT in the classifier) and TS-44's
sticky suppression. Silently dropping them by delegating to the classifier is
named as the failure mode.

Acceptance requires a BEHAVIOURAL App test, not the source-text pin route 2
settled for (#292).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/plans/2026-08-02-placement-cutover.md

@@ -254,6 +254,26 @@ same commit) → docs/handoff commit. No workarounds; no fused slices.
   6 (drops + split-recovery marking), 7 (residual pickup/parent/delete
   polish). — route 2 COMPLETE AND USER-ACCEPTED 2026-08-03 (`9966b531`);
   routes 3/4/5/6/7 remain OPEN.**
+  **Route 4 SPLIT into 4a and 4b (user-directed 2026-08-03).** Scoping
+  ([`2026-08-03-c4-route-4-scoping.md`](../research/2026-08-03-c4-route-4-scoping.md))
+  put whole-route 4 at 1,500-2,500 production lines against a stated ~400
+  budget, so it is split to keep each landing reviewable:
+  - **4a — the steady state.** The classifier's `Interpolate` (contact,
+    `PlayerDistance < 96 m`) and `NoPositionOperation` (no contact) branches.
+    NEITHER runs a `SetPosition`, so 4a has no deferred-cell park, no
+    service-window work, and no placement-allocation exposure. Fixes two of the
+    three unfiled divergences (the NPC airborne hard-snap that ignores the wire
+    `IsGrounded` bit; `ConstrainTo` armed before the operation instead of
+    after). Highest visible value — this is what makes creatures move smoothly.
+  - **4b — the edges.** `SetPosition` / `SetPositionSimple`: teleport, far-snap
+    (>= 96 m), and cell-less first placement. This is where the parks, the
+    Position-time service-window guard, #277's broken bound, N3 (headless never
+    calls `RetryPending`), and the third divergence (`ConstrainTo` never armed
+    on the remote teleport branch) all live.
+  Note the route-4 Create half is ALREADY DONE (C3b/C3c); the remaining work is
+  steady-state remote Position plus the deletions. AP-131 is NOT retired by
+  either sub-slice — see the scoping doc for why route 4 alone cannot.
+  4a contract: [`2026-08-03-c4-route-4a-contract.md`](../research/2026-08-03-c4-route-4a-contract.md).
   **Route 2 connected gate PASSED (user, 2026-08-03).** Provoked with the
   retail `@pklite` entry-collision bump (`69ba9486` — the only reachable ACE
   trigger for `ObjectForcePosition`; admin teleports advance `ObjectTeleport`

docs/research/2026-08-03-c4-route-4a-contract.md

@@ -0,0 +1,155 @@
+# C4 route 4a — remote steady-state Position: pinned contract (2026-08-03)
+
+Route 4 split into 4a/4b by user direction after scoping put the whole route at
+1,500-2,500 lines against a ~400 budget. Scoping:
+[`2026-08-03-c4-route-4-scoping.md`](2026-08-03-c4-route-4-scoping.md).
+
+**4a is the steady state: the two classifier branches that perform NO
+`SetPosition`.** Everything that parks, teleports, or snaps is 4b.
+
+## Scope — exactly two classifier branches
+
+`RuntimeAuthoritativePositionRouteClassifier.ClassifyAcceptedPosition`
+(`:393-472`), remote/projectile arms:
+
+| Branch | Condition | Runs SetPosition? | In 4a? |
+|---|---|---|---|
+| `NoPositionOperation` | `!effectiveContact` (wire `IsGrounded == false`) | no | **YES** |
+| `Interpolate` | contact, `PlayerDistance < 96 m` | no | **YES** |
+| `SetPositionSimple` | contact, `PlayerDistance >= 96 m` | yes | no — 4b |
+| `SetPosition` | `TeleportAdvanced` or `CommittedCellId == 0` | yes | no — 4b |
+
+Because neither 4a branch performs a placement, 4a has **no deferred-cell park,
+no service-window guard, no allocation exposure, and no interaction with
+`Forget`-on-every-accepted-Position.** That is the entire reason for the split;
+do not drag any of it in.
+
+## Staged cutover — the one sanctioned dual path
+
+4a routes the two no-placement branches through the new Runtime seam and leaves
+the other two on the legacy App path until 4b. **This is a staged cutover, not a
+duplicate authority**, and it is only sanctioned under these conditions:
+
+1. The discriminator is the CLASSIFIER ITSELF, not a heuristic, a flag, or a
+   guess. One classification, one owner, mutually exclusive by construction.
+2. For a classification 4a owns, the legacy path must not run **at all** — not
+   partially, not "just the render write". Route 2's original defect was exactly
+   a second writer running after the canonical one.
+3. The fallback is temporary and 4b deletes it. Record that in the code comment
+   at the branch point, with a pointer to this contract.
+
+If you find yourself needing a condition beyond "what did the classifier say",
+STOP and report — that means the seam is wrong.
+
+## Retail truth (verify each yourself; do not trust this table)
+
+`CPhysicsObj::MoveOrTeleport` @0x00516330 (pseudo-C 284304):
+
+- **Airborne no-op** @0x0051638E / @0x0051636D: `arg4 == 0` (the wire
+  `has_contact` bit) -> **return 0. Nothing is written at all.**
+- **Near interpolate** @0x005163AF: `player_distance < 96f` ->
+  `InterpolateTo(arg2, IsMovingTo())`. No body write.
+- `player_distance` is retail's own field — distance to the local player.
+
+`SmartBox::HandleReceivedPosition` @0x00453FD0, remote branch:
+
+- `unset_parent` @0x00454129 unconditional; `SetPlacementFrame` @0x00454142
+  gated on `!HasAnims`.
+- **`ConstrainTo` @0x00454272 runs AFTER `MoveOrTeleport` returns nonzero**,
+  anchored to `&arg2->m_position` — the object's own position read live, i.e.
+  POST-move. It does not run when `MoveOrTeleport` returned 0 (the airborne
+  no-op).
+
+## The two divergences 4a must fix
+
+**D1 — the NPC airborne branch ignores the wire bit.**
+`LiveEntityNetworkUpdateController.cs:1819-1823` hard-snaps
+`Body.Position`/`Orientation` for NPCs and branches on the client-tracked
+`rmState.Airborne` flag, never consulting `update.IsGrounded`. Retail writes
+nothing. Player remotes already behave correctly (`:1590-1599`); NPCs do not.
+No register row exists.
+
+**D2 — `ConstrainTo` is armed before the operation, unconditionally.**
+`:1522-1529` arms it ahead of the branch, so it fires on the airborne no-op
+retail skips and anchors to the PRE-move position. Retail arms it after, only on
+a nonzero return, anchored post-move. No register row exists.
+
+(The third divergence — `ConstrainTo` never armed on the remote teleport branch
+— is on a 4b path. Leave it; 4b owns it.)
+
+## Duplicate authorities 4a deletes
+
+Only the parts reachable from the two 4a branches:
+
+- The generic tail's remote writes (`entity.SetPosition` / `ParentCellId` /
+  `Rotation` / `RebucketLiveEntity`) for a remote whose classification is
+  `Interpolate` or `NoPositionOperation`. The committed result must reach the
+  render entity through the existing placement-projection sink instead — the
+  same substitution route 2 made.
+- The player-remote near/far routing at `:1653-1702` and the NPC copy at
+  `:1826-1871` — **the near half only.** Each has its own duplicated copies of
+  `MaxPhysicsDistance = 96f` and `BodySnapThreshold = 4f`; the far half stays
+  until 4b.
+- The airborne no-op blocks at `:1590-1599` (player) and `:1819-1823` (NPC).
+- The unconditional `ConstrainTo` at `:1522-1529`.
+
+Do NOT touch `RemoteTeleportController`, `RemoteTeleportPlacement`, or the
+`remotePlacementRequired` path — all 4b.
+
+## Load-bearing acdream additions that must survive
+
+**AP-87** (`retail-divergence-register.md:242`) — the `bodyToTarget > 4 m` and
+`!willBeDrTicked` snap conditions on the near branch are NOT in retail and NOT
+in the classifier. They are load-bearing: they prevent the #184
+invisible-but-solid monster. Either carry them explicitly as an acdream policy
+layer over the retail classification, or retire them with live evidence and the
+register row deleted in the same commit. **Silently dropping them by delegating
+to the classifier is the failure mode.** Say which you chose.
+
+**TS-44** (`:1792-1801`) — sticky-melee suppression of the NPC snap. Same rule.
+
+## Contract
+
+1. One Runtime owner executes the accepted remote Position for the two 4a
+   classifications; App projects the result.
+2. Both hosts drive the identical Runtime entry point.
+3. The interpolation queue (`RemoteMotion.Interp`) stays the owner of near
+   motion — 4a routes to it, it does not replace it. All of it is already in
+   Runtime (J5.5); no assembly boundary is crossed.
+4. Stale sequences, GUID reuse, incarnation change, and generation change cannot
+   commit old state. N entities, so per-entity currency — route 2's `_pending`
+   was one slot and `RetainPending` threw on a second; that shape does not
+   transfer.
+5. The airborne branch writes NOTHING (retail returns 0). Not the body, not the
+   render entity, not the cell.
+6. `ConstrainTo` moves to after the operation, anchored post-move, and does not
+   run on the airborne branch.
+7. No behaviour change to the far, teleport, or cell-less branches.
+
+## Acceptance
+
+- Focused Runtime tests for both branches, per-entity currency, and the AP-87 /
+  TS-44 decision.
+- App tests proving the generic tail no longer double-writes a remote on a 4a
+  classification — **behavioural, not a source-text pin.** Route 2's equivalent
+  was source-pinned and that gap is filed as #292; do not repeat it.
+- Complete Release suite green. Baseline at the time of writing:
+  **10,904 passed / 4 skipped / 0 failed**. Known flake #302
+  (`PortalProjectionTests.ClipToRegion_FrameOwnedStore_ReusesExactResultArray`,
+  ~1 in 6) — re-run, do not chase.
+- **Connected (user-gated), and unlike route 2 this is trivial:** stand still,
+  have a second character walk and run in a circle 5-15 m away, turning in
+  place. Motion must be continuous and smooth with no per-packet stepping. Then
+  have them jump and jump off a ledge — a clean parabola, clean landing, no
+  mid-air correction, and critically no invisible-but-solid body (the #184 /
+  AP-87 signature). Then pull a drudge, let it chase, melee it, let it die.
+- Divergence ledger: D1 and D2 retired by fix in the same commit. AP-131 stays.
+  AP-87 / TS-44 either stay with justification or are retired with evidence.
+
+## Budget
+
+Stated up front so it can fail: **4a should be well under 400 production
+lines**, because it adds no placement machinery — it routes two no-op
+classifications and deletes their duplicates. If it exceeds that, stop and
+report before continuing; that would mean the split did not actually isolate the
+cheap half and 4b needs re-planning too.