feat(physics): C4 route 4a — remote steady-state Position through the seam

Routes the classifier's two NO-PLACEMENT remote branches — Interpolate
(contact, PlayerDistance < 96 m) and NoPositionOperation (no contact) — through
a Runtime-owned seam, and fixes the two divergences they carried. Teleport,
far-snap and cell-less stay on the legacy App path; 4b owns them.

Route 4 was split into 4a/4b after scoping put the whole route at 1,500-2,500
lines against a ~400 budget. 4a's branches perform no SetPosition, so this slice
carries no deferred-cell park, no service-window guard and no allocation
exposure — which is what made the split worth doing.

Divergences fixed, both previously unfiled:

* D1 — the NPC airborne branch hard-snapped Body.Position/Orientation and
  branched on the client-tracked rmState.Airborne, never consulting the wire
  IsGrounded bit. Retail's MoveOrTeleport @0x00516330 returns 0 at 0x0051636D
  and writes nothing. Player remotes were already correct; NPCs were not.
* D2 — ConstrainTo was armed before the operation, unconditionally, so it fired
  on the airborne no-op retail skips and anchored to the PRE-move position.
  Retail arms it at 0x00454272, only when MoveOrTeleport returns nonzero,
  anchored to &arg2->m_position read live, i.e. post-move.

AP-87 and TS-44 were carried deliberately, not delegated away. AP-87's three
conditions — including firstUp, which one round silently dropped — are preserved
as an explicit acdream policy layer applied AFTER the classifier commits to
Interpolate; the two previously separate player/NPC copies are now one. TS-44
stays an NPC-only caller gate; extending sticky suppression to player remotes has
no retail basis and no live evidence, so it was declined rather than absorbed.

Landing is explicitly carved out of 4a's ownership on both arms. A landing packet
classifies Interpolate, so an ordering slip would ENQUEUE a body that must PLANT
and a creature knocked off a ledge would glide down over a packet interval. The
carve-out is a named entry point returning AirborneSnap/SteadyStateInterpolate/
Legacy precisely so the PRECEDENCE is observable and testable rather than implied
by statement order — that is how the slip happened once and was caught.

The player/NPC asymmetry on landing is real and NOT resolved here: retail draws
no such distinction, but converging them is a behaviour decision needing its own
evidence. Filed into the 4b plan.

Register: AP-135 filed for the two bookkeeping writes the airborne branch
deliberately retains (rmState.CellId, LastServerPos/Time) — not retail's model,
but load-bearing for our catch-up sweep and staleness timer, and verified not to
be a canonical cell commit for ordinary remotes. AP-87 and TS-44 rewritten to
describe the code.

Honest remainder: App still owns branch selection, the airborne return, the cell
write, the entity write and the shadow publish, and headless satisfies "both
hosts drive the identical entry point" only vacuously since it returns early for
remotes. That is written into the 4b bullet rather than left implicit.

Cost: 364 non-comment production lines, 91% of the ~400 budget — the split did
isolate the cheap half, but not by much. Do not carry "well under" into 4b's
scoping.

Gates: complete Release solution 10,938 passed / 4 skipped / 0 failed (pre-4a
baseline 10,909). Four review rounds; the first three each introduced a new
behavioural defect while fixing another, and each left a comment asserting
behaviour that no longer matched — the final round's precedence matrix was
traced cell-by-cell against HEAD with only the D1-intended difference. App tests
call production entry points against a real WorldEntity and real classifier
output, closing route 2's #292 gap rather than repeating it.

Connected acceptance NOT run — needs a live second character.

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

src/AcDream.Runtime/Entities/RuntimeInitialCreateContinuationExecutor.cs

@@ -1904,46 +1904,26 @@ internal sealed class RuntimeInitialCreateContinuationExecutor
         }
         else
         {
-            var authority = new RuntimeAuthoritativePositionAuthority(
-                CurrentGeneration(),
-                key,
-                canonical.PositionAuthorityVersion,
-                update.PositionSequence,
-                action.PreviousTeleportSequence,
-                action.AcceptedTimestamps.Teleport,
-                action.PositionDisposition);
-
-            // Round 3 A3: contact comes SOLELY from the retained wire
-            // packet's own IsGrounded bit (PositionPack bit 0x4,
-            // server-asserted contact at admission time) - never a live
-            // body query, never an Inputs fallback.
-            bool hasContact = update.IsGrounded;
-            // Round 3 B5: HasAnimations is the SAME data-driven proxy for
-            // every position source, SameIncarnationCreate included - no
-            // PositionSource short-circuit. Round 4 R4-13: fall back to the
-            // nested PhysicsSpawnData's own MotionTableId when the
-            // top-level snapshot field is null (WeenieDescription/ObjDesc
-            // merges only ever populate one of the two, depending on
-            // which stage last touched appearance vs description).
-            bool hasAnimations = (canonical.Snapshot.MotionTableId
-                    ?? canonical.Snapshot.Physics?.MotionTableId) is { } motionTableId
-                && motionTableId != 0u;
-
-            var request = new RuntimeAcceptedPositionRouteRequest(
-                authority,
-                entityKind,
-                action.PositionSource,
-                update.Position,
-                update.PlacementId,
-                update.Velocity,
-                canonical.FullCellId,
-                hasContact,
-                inputs.PlayerDistance,
-                inputs.UsePositionFromServer,
-                hasAnimations,
-                new RuntimePositionPlacementFacts(
-                    canonical.FinalPhysicsState,
-                    canonical.Snapshot.SetupTableId is not null));
+            // Round 3 A3 / B5 and Round 4 R4-13 (contact from the retained
+            // wire packet's own IsGrounded bit only; the data-driven
+            // HasAnimations proxy with its PhysicsSpawnData fallback) now
+            // live in the ONE shared request builder, which C4 route 4a's
+            // remote classification also uses - see
+            // RuntimeAcceptedPositionRouteRequests for why a second
+            // hand-written copy of this construction is not allowed.
+            RuntimeAcceptedPositionRouteRequest request =
+                RuntimeAcceptedPositionRouteRequests.Build(
+                    CurrentGeneration(),
+                    canonical,
+                    key,
+                    update,
+                    entityKind,
+                    action.PositionSource,
+                    action.PositionDisposition,
+                    action.PreviousTeleportSequence,
+                    action.AcceptedTimestamps.Teleport,
+                    inputs.PlayerDistance,
+                    inputs.UsePositionFromServer);
 
             route = RuntimeAuthoritativePositionRouteClassifier.ClassifyAcceptedPosition(request);