fix(physics): classify before merge on every steady-state Position (C5b, #275, AP-131/AD-60)

The steady-state accepted-Position merge did two things retail never does,
on every single Position packet: it installed the wire placement frame and
unparented unconditionally, and it derived the record's FullCellId from
bare wire acceptance. Both are now correct, and they land together - a
half-flipped intermediate (classified flags with the wire stamp, or vice
versa) is exactly the mixed-residency state this campaign keeps paying for.

WHY the flags need no route. SmartBox::HandleReceivedPosition @0x00453FD0
decides both pre-placement writes BEFORE MoveOrTeleport is consulted: Gate A
@0x0045400C returns @0x0045409D ahead of unset_parent @0x00454129 and ahead
of the HasAnims SetPlacementFrame gate @0x00454137. Neither gate reads the
near/far/teleport classification. So the two flags are a pure function of
(disposition, hasAnimations) and are computable inside the merge, pre-merge,
with no signature change, no route construction and no playerDistance - the
scoping's ~150-400-line route-plumbing estimate over-counted because it did
not see this. That truth table IS
RuntimeAuthoritativePositionRouteClassifier.ClassifyAcceptedPosition's own
ApplyPlacementFrameBeforeRouting/UnparentBeforeRouting rows; the classifier
stays the oracle and the equality is pinned by test, not by a shared path,
so each computation remains separately sabotage-verifiable.

WHY the cell is withheld. HandleReceivedPosition reads the wire objcell_id
into a LOCAL @0x00453FE3 and hands it only to BlipPlayer / TeleportPlayer /
MoveOrTeleport / ConstrainTo; it never assigns the object's cell. The
object's cell moves inside the placement family (SetPositionInternal
@0x00515BD0 to set_cell, enter_world) or per-frame transit, and nowhere
else. The continuation executor has encoded that rule since the executor
slice; this caller now matches it verbatim.

WHAT DELIBERATELY SURVIVES. Two steady-state wire-cell writers stay,
downstream of the merge and outside the classification window: the
OnPosition prologue rebucket (W2, into CommitRebucket), which is also the
local player's own cell-freshness path, and the post-routing wire-cell adopt
for non-placing arms (W3, AP-135). Gating W2 "for symmetry" would freeze the
player's canonical cell between teleports and #319's child-cell equality
would inherit the freeze. AD-60's rewrite names both so the retirement
cannot be misread as "wire acceptance never changes residency anywhere".

REGISTER. AP-131 RETIRED - the unconditional literals no longer exist; the
caller was corrected, not deleted, so the row's own "deleted at the
production cutover" framing is overtaken. AD-60's legacy half RETIRED and
the row REWRITTEN rather than deleted, naming W2/W3 (route 4b-3's D8
precedent: a silent whole-row deletion would hide surviving channels).
AP-130 amended - the merge consumes the same static HasAnimations proxy,
deliberately not escalated to a live animation-queue read. AP-146 and #320
amended - their "accepted inbound Position (RefreshSnapshot into
RuntimeEntityRecord.cs:234)" local-player cell writer is now the generic
tail's CommitRebucket, and a ForcePosition (which returns before that tail)
is placement-receipt-authoritative. #275 closed.

HEADLINE BEHAVIOURAL DELTA, stated once: a refused or contended local
ForcePosition now leaves FullCellId at the last committed cell where the
merge used to stamp the refused packet's wire cell. Retail cannot refuse
(AD-62) and its body keeps its last placed cell, so the new shape is the
retail-reachable one.

THREE CONSUMER SITES THE CONTRACT'S BLAST-RADIUS SURVEY MISSED, all
D2-caused, all found by the suite rather than by reading, all intended
semantics rather than regressions (recorded in the contract's new section
14):
(1) DatLiveEntityProjectionMaterializer's self-projection branch reads
    FullCellId inside OnPosition's prologue recovery, ahead of W2. It now
    correctly declines to project from an unplaced wire claim; production
    installs the bucket at W2 in the same call (verified: no return between
    the recovery call and W2 is conditioned on IsSpatiallyProjected or
    FullCellId). Two hydration tests asserted the bucket at the recovery
    boundary and now drive the production W2 step - the same shape as trap
    T2, one layer up.
(2) ProjectileController.SyncPresentationFromResolvedBody writes
    ParentCellId = record.FullCellId. On a refused missile placement that is
    now the committed source cell. The MAJOR-1 invariant is unchanged and is
    now asserted as the identity it always meant rather than as a wire-cell
    constant.
(3) The merge's Rebucketed ternary does NOT become always-Updated as the
    contract predicted, and is deliberately kept: the
    Forget(restoreCancelledPark: true) above it can roll a wakeable
    lost-cell park back, and RestoreParkWithdrawal restores canonical
    residency. That is a real cell edge produced inside this method by a
    placement owner.

TEST-COUNT RECONCILIATION. Baseline measured at this HEAD by stashing the
change: Runtime.Tests 1176, App.Tests 4135 (4132 passed / 3 skipped),
solution 11,106 passed / 4 skipped - matching the recorded figure at
6921a027 exactly. Post-change: Runtime.Tests 1195, App.Tests 4135 unchanged,
solution 11,125 passed / 4 skipped / 0 failed. Net +19, entirely new Runtime
tests: 3 facts plus a 12-row matrix theory in
InboundPhysicsStateControllerTests, 1 fact plus a 2-row theory in the new
RuntimeSteadyStatePositionMergeTests, and 1 fact in
RuntimeAcceptedPositionDriveControllerTests. No test was deleted; five
existing tests were rewritten in place, never delete-only. No new skip; none
of #302/#308/#321 appeared.

SABOTAGE VERIFICATIONS (each new discriminating test, both directions;
production line broken, suite run, line restored):
  installPlacementFrame (!force && !hasAnimations) to (!force)
    5 fail: ApplyOnAnimatedEntity_NeverInstallsTheWirePlacementFrame plus
    the 4 animated non-force matrix rows.
  installPlacementFrame to false
    6 fail: ApplyOnNonAnimatedEntity_InstallsTheWirePlacementFrame,
    PositionPlacementAbsentAndPresentZeroBothApplyRetailZero plus the 4
    non-animated non-force matrix rows.
  clearParent (!force) to true
    3 fail: ForcePositionOnParentedLocalPlayer_RetainsTheParentAttachment
    plus the 2 force+parented matrix rows.
  clearParent (!force) to false
    4 fail: the 4 Apply+parented matrix rows.
  refreshPosition false to acceptedPosition
    4 fail: AcceptedPosition_WithholdsTheWireCellAtTheMergeBoundary,
    ContendedForcePosition_WritesNoResidencyAnywhere,
    ReentrantNewerPositionDuringPickupDiscardSuppressesStalePickupDelta,
    MissileFarRefused_...ParentCellIdAgreesWithCommittedCell. Confirmed a
    second time by the baseline measurement above, where the withhold test
    was the sole red.
  CommitRebucket publishes Updated instead of Rebucketed
    2 fail: both parent classes of
    CellChangingAcceptedPosition_ConservesOneRebucketAndOneChildPropagation.
  RuntimeEntityDirectory.SetFullCell drops PropagateFullCellToChildren
    2 fail: the same two rows.
T4 respected: the ForcePosition placement-frame half is inert
(appliedPlacement keeps old.PlacementId under either flag value), so the
force row's discriminating assertion is parent retention, never the frame.

NOT DONE, deliberately: the executor is still not wired into the
steady-state path (#275's alternative branch); W2/W3 are untouched; no probe
added or stripped; AP-130's proxy not escalated; no while-here unification
of the two merge callsites. No automated OnPosition-level test drives the
full pickup / drop / reproject sequence (no fixture covers pickup at that
layer); the contract's connected gate recipe item 1 is the positive evidence
for it and has NOT been run - this commit is not connected-gated.

Contract: docs/research/2026-08-05-c5b-contract.md (committed here, with its
section 14 implementation outcome appended).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/AcDream.Runtime.Tests/Session/RuntimeAcceptedPositionDriveControllerTests.cs

@@ -255,6 +255,81 @@ public sealed class RuntimeAcceptedPositionDriveControllerTests
         }
     }
 
+    /// <summary>
+    /// C5b (#275) proof obligation 4 — the assertion that makes AD-60's
+    /// legacy-half retirement mean something. A local ForcePosition whose
+    /// drive execution CONTENDS performs no placement, so nothing may write
+    /// residency for that packet: the merge no longer stamps the wire cell
+    /// (C5b), there is no placement receipt, and the OnPosition generic tail
+    /// (W2) is never reached because the force path returns before it.
+    /// <c>FullCellId</c> is the pre-packet committed cell throughout.
+    ///
+    /// <para>
+    /// Retail cannot refuse at all (AD-62), and its body simply keeps the
+    /// last cell <c>set_cell</c> gave it — so the surviving shape here is the
+    /// retail-reachable one. Before C5b the merge stamped the refused
+    /// packet's wire cell, which is the AD-60 legacy-half divergence.
+    /// </para>
+    /// </summary>
+    [Fact]
+    public void ContendedForcePosition_WritesNoResidencyAnywhere()
+    {
+        using StartedRuntime started = StartRuntime();
+        GameRuntime runtime = started.Runtime;
+        (RuntimeEntityRecord record, PlayerMovementController controller) =
+            EnterLocalPlayer(runtime);
+        uint committedCellBefore = record.FullCellId;
+        Assert.NotEqual(0u, committedCellBefore);
+
+        // A wire cell that is deliberately NOT the committed one, so the
+        // pre-C5b stamp would have been observable.
+        const uint wireCell = SpawnLandblock | 0x0002u;
+        Assert.NotEqual(wireCell, committedCellBefore);
+        WorldSession.EntityPositionUpdate wire = ForceUpdate(
+            new Vector3(30f, 32f, SpawnHeight),
+            landblockId: wireCell);
+        (PositionTimestampDisposition disposition, AcceptedPhysicsTimestamps timestamps) =
+            MergeAccepted(runtime, controller, wire);
+        Assert.Equal(PositionTimestampDisposition.ForcePosition, disposition);
+
+        // (1) The merge itself wrote no residency.
+        Assert.Equal(committedCellBefore, record.FullCellId);
+
+        RuntimeEntityPlacementToken displaced = runtime.EntityObjects.Physics
+            .SetPosition.TryBeginExclusiveAuthoredPlacement(
+                record,
+                record.PositionAuthorityVersion,
+                RuntimeSetPositionOperationKind.LocalAuthoritative);
+        Assert.True(displaced.IsValid);
+        RuntimeAcceptedPositionDriveController drive =
+            CreateAcceptedPositionDrive(runtime, out List<byte[]> gameActions);
+
+        Assert.Equal(
+            RuntimeAcceptedPositionExecutionStatus.Contention,
+            drive.TryExecuteAcceptedLocalPosition(
+                record,
+                wire,
+                disposition,
+                timestamps,
+                timestamps.PreviousTeleport));
+
+        // (2) No receipt, so no residency change from the execution either.
+        Assert.Equal(committedCellBefore, record.FullCellId);
+        Assert.Empty(gameActions);
+
+        // (3) And nothing rebounds it on the settle.
+        drive.Advance();
+        Assert.Equal(committedCellBefore, record.FullCellId);
+
+        RuntimePlacementCancellationReceipt cancellation = runtime.EntityObjects
+            .Physics.SetPosition.ForgetExactPlacement(displaced);
+        if (cancellation.IsValid)
+        {
+            runtime.EntityObjects.Physics.SetPosition
+                .PublishCancellation(cancellation);
+        }
+    }
+
     [Fact]
     public void DeferredCell_ParksThenCommitsAndNeverDoubleAcksAfterTheCollisionGenerationWakes()
     {