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`