feat(physics): C4 route 2 — ForcePosition through the canonical placement

A local-player ForcePosition had TWO independent writers for one accepted
packet: LocalForcePositionTransaction snapped the physics body
(PlayerMovementController.BlipPosition, a raw SnapToCell with no collision
resolve), while LiveEntityNetworkUpdateController's generic tail separately
wrote position/cell/rotation to the render WorldEntity from the raw wire and
rebucketed it. Two stores, one packet — the divergence class 670f307c fixed on
the remote path. The outbound AutonomousPosition ack also fired BEFORE any
canonical commit existed: we told ACE "got it, I'm here" before deciding where
"here" was, and the trailing isCurrent() could only suppress the continuation,
never recall the packet.

RuntimeAcceptedPositionDriveController is now the one Runtime-owned seam. Both
hosts call the identical TryExecuteAcceptedLocalPosition; App and headless
project the committed result through the existing placement projection sink
(LiveEntityRuntime.TryApplyRuntimePlacementPlace already performed the same
four writes, from committed state rather than a wire guess).

Retail: SmartBox::HandleReceivedPosition @0x00453FD0's FORCE_POSITION branch is
get_heading -> Frame::set_heading -> SmartBox::BlipPlayer @0x00453940 -> stamp
POSITION_TS -> SendPositionEvent @0x00454091 -> return @0x0045409D. BlipPlayer
is CPhysicsObj::SetPositionSimple @0x005162B0 with flags 0x1012
(Teleport|Slide|SendPositionEvent) — a real collision-resolving SetPosition,
not a snap. The pinned classifier already encoded this exactly.

Named behaviour changes:

* The ack is now an OUTPUT of the committed route, fired strictly after the
  canonical commit and exactly once per accepted force packet.
* The ForcePosition route no longer re-arms the constraint leash. The force
  branch returns at 0x0045409D, ahead of all three ConstrainTo sites
  (0x00454272, 0x0045418A, 0x004541EC); the old re-arm cited retail's "Player,
  normal" branch, which BlipPlayer is not on. The teleport, CommitPreparedPosition
  and first-entry callers legitimately still constrain and are untouched.
* A force correction that terminates WITHOUT committing still sends its
  position event and is not retried — retail's BlipPlayer discards
  SetPositionSimple's SetPositionError return and acks unconditionally.

A single _pending funnel owns the in-flight placement, deciding on the token's
PositionAuthorityVersion against the record's: equal -> clear; advanced with the
newest accepted event still a force -> re-issue, re-classified; advanced to an
ordinary Apply -> clear, since newer server truth owns that pose. This closes a
double-apply/double-ack and a silently-dropped correction that two earlier
iterations of this slice each introduced.

AD-62 records the residual: a ForcePosition our async collision publication
cannot carry to a committed placement is not re-applied. Retail has no park —
its world is fully resident and its placement synchronous — so the state is
unreachable there. AP-131 is NOT retired; its legacy Position caller is route 4.

Deleted: LocalForcePositionTransaction, PlayerMovementController.BlipPosition,
HeadlessSessionWorldProjection.BlipLocalPlayer.

Gates: complete Release solution 10,858 passed / 4 skipped / 0 failed (baseline
10,844/4/0). Two independent Opus reviews (retail-conformance and
architecture/adversarial) PASS on the final diff after three FAIL rounds; every
intermediate state was fully green, so the suite caught none of the four real
defects. Connected acceptance is NOT run: nothing a user can do makes ACE emit
a ForcePosition without retail's @pklite, which acdream does not implement — see
docs/research/2026-08-03-c4-route-2-visual-gate.md.

Known gap, recorded not claimed: the plan's acceptance item 2 is unmet. The App
double-write check is a source pin, and "the committed projection moves the
render entity" is uncovered at any layer (#292). Filed alongside: #286-#291,
#293-#296.

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

src/AcDream.App/Composition/SessionPlayerComposition.cs

@@ -573,6 +573,21 @@ internal sealed class SessionPlayerCompositionPhase
                             .RegisteredAuthoredPayload
                         : RuntimeLocalPlayerShadowDisposition.ProvenShapeless);
             });
+        // C4 route 2 (2026-08-03): the graphical accepted-Position drive
+        // controller — the Runtime-owned execution seam for a ForcePosition
+        // on the already-live local player. Constructed once per session
+        // route alongside firstEntryDrive; both share the SAME entity
+        // lifetime, collision source, and clock.
+        var acceptedPositionDrive = new RuntimeAcceptedPositionDriveController(
+            d.EntityObjects,
+            d.Runtime.Clock,
+            firstEntryCollision,
+            d.PlayerOutbound,
+            () => d.Runtime.Generation,
+            () => d.PlayerIdentity.ServerGuid,
+            () => d.PlayerController.Controller,
+            () => d.Character.UsePositionFromServer,
+            () => liveSessionSource.CurrentSession);
         var hydration = new LiveEntityHydrationController(
             live.LiveEntities,
             d.EntityObjects,
@@ -591,7 +606,8 @@ internal sealed class SessionPlayerCompositionPhase
             deletion,
             dormantLiveEntities,
             d.Options.DumpLiveSpawns ? d.Log : null,
-            firstEntryDrive);
+            firstEntryDrive,
+            acceptedPositionDrive);
         bindings.Adopt(
             "landblock-loaded hydration",
             live.LandblockLoaded.Bind(hydration));
@@ -637,6 +653,7 @@ internal sealed class SessionPlayerCompositionPhase
             liveSessionSource,
             localPhysicsTimestamps.Publish,
             d.MovementDiagnostics,
+            acceptedPositionDrive,
             worldDropProjection);
         var liveness = new LiveEntityLivenessController(
             live.LiveEntities,
@@ -959,7 +976,8 @@ internal sealed class SessionPlayerCompositionPhase
                 live.RenderSceneShadow,
                 live.PlacementProjection,
                 placementProjectionRetry,
-                firstEntryDrive),
+                firstEntryDrive,
+                acceptedPositionDrive),
             liveSessionCommands,
             d.Log);
         LiveSessionHost sessionHost = sessionRuntimeFactory.Create(