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>

tests/AcDream.App.Tests/Physics/LiveEntityNetworkBranchRoutingTests.cs

@@ -1,3 +1,4 @@
+using System.Text.RegularExpressions;
 using AcDream.App.Physics;
 
 namespace AcDream.App.Tests.Physics;
@@ -46,51 +47,97 @@ public sealed class LiveEntityNetworkBranchRoutingTests
         Assert.Equal(["projectile", "canonical", "ordinary"], calls);
     }
 
-    [Fact]
-    public void ForcePosition_BlipsAndAcknowledgesExactlyOnce()
+    // C4 route 2 (2026-08-03): LocalForcePositionTransaction and its
+    // ForcePosition_* coverage here are RETIRED, not adapted — the class is
+    // deleted outright (docs/research/2026-08-03-c4-route-2-contract.md
+    // §"The contract" item 2). Its three jobs (ownership validation, the
+    // blip/commit, and the exactly-once ack including the displaced-
+    // authority case its trailing isCurrent() covered) are now properties of
+    // the Runtime-owned RuntimeAcceptedPositionDriveController and are tested
+    // there: tests/AcDream.Runtime.Tests/Session/RuntimeAcceptedPositionDriveControllerTests.cs.
+
+    /// <summary>
+    /// R8 review fix (2026-08-03): source pins for the generic-tail
+    /// double-write guard in <c>LiveEntityNetworkUpdateController.OnPosition</c>.
+    /// A full behavioral fixture is impractical here for the SAME reason
+    /// <c>C3cF1ProductionWiringTests</c> gives — the controller's dependency
+    /// set is composition-only (67+ collaborators wired only by
+    /// <c>SessionPlayerComposition</c>) — so this follows that file's exact
+    /// established pattern: assert the STRUCTURE of the production source
+    /// rather than construct the class. The Runtime-level behavioral
+    /// coverage for the seam itself lives in
+    /// RuntimeAcceptedPositionDriveControllerTests; these pins are what stop
+    /// this App-layer call site from silently reintroducing the retired
+    /// duplicate-write authority (the deleted LocalForcePositionTransaction
+    /// pair + the generic render-tail writing the SAME accepted Position a
+    /// second time — 670f307c's divergence class).
+    /// </summary>
+    public sealed class LiveEntityNetworkUpdateControllerForcePositionWiringTests
     {
-        int currentChecks = 0;
-        int blips = 0;
-        int acknowledgements = 0;
+        [Fact]
+        public void LocalForcePositionTransactionIsNeverCalledFromThisFile()
+        {
+            string source = ReadSource("LiveEntityNetworkUpdateController.cs");
 
-        bool completed = LocalForcePositionTransaction.Apply(
-            isForcePosition: true,
-            () => { currentChecks++; return true; },
-            () => blips++,
-            () => acknowledgements++);
+            // The name may still appear in a comment explaining what
+            // replaced it (contract §"the deleted LocalForcePositionTransaction");
+            // what must be gone is any actual call into it.
+            Assert.DoesNotContain(
+                "LocalForcePositionTransaction.Apply(",
+                source,
+                StringComparison.Ordinal);
+        }
 
-        Assert.True(completed);
-        Assert.Equal(2, currentChecks);
-        Assert.Equal(1, blips);
-        Assert.Equal(1, acknowledgements);
-    }
+        [Fact]
+        public void GenericTailWriteIsNeverDuplicatedForTheLocalForcePositionPath()
+        {
+            string source = ReadSource("LiveEntityNetworkUpdateController.cs");
 
-    [Fact]
-    public void ForcePosition_AcknowledgementInvalidationStopsTheTail()
-    {
-        bool current = true;
-        int acknowledgements = 0;
+            // The generic render-tail's WorldEntity write is the ONE
+            // remaining writer of an accepted Position — it must serve
+            // remotes only, never a second local-player write alongside the
+            // Runtime-committed one.
+            Assert.Single(
+                Regex.Matches(source, @"entity\.SetPosition\(worldPos\);")
+                    .Cast<Match>());
+        }
 
-        bool completed = LocalForcePositionTransaction.Apply(
-            isForcePosition: true,
-            () => current,
-            () => { },
-            () => { acknowledgements++; current = false; });
+        [Fact]
+        public void CommittedOrDeferredCellReturnsBeforeReachingTheGenericTail()
+        {
+            string source = ReadSource("LiveEntityNetworkUpdateController.cs");
 
-        Assert.False(completed);
-        Assert.Equal(1, acknowledgements);
-    }
+            // The Committed/DeferredCell branch must still return
+            // immediately after its two preserved side effects — a missing
+            // `return` here would fall through into the generic tail below
+            // and resurrect the double-write.
+            Assert.Matches(
+                new Regex(
+                    @"ObserveAcceptedLocalPosition\(\s*"
+                        + @"update\.Position\.LandblockId\);\s*return;",
+                    RegexOptions.Singleline),
+                source);
+        }
 
-    [Fact]
-    public void OrdinaryPositionDoesNotBlipOrAcknowledge()
-    {
-        int calls = 0;
+        private static string ReadSource(string fileName)
+        {
+            DirectoryInfo? directory = new(AppContext.BaseDirectory);
+            while (directory is not null)
+            {
+                if (File.Exists(Path.Combine(directory.FullName, "AcDream.slnx")))
+                {
+                    return File.ReadAllText(Path.Combine(
+                        directory.FullName,
+                        "src",
+                        "AcDream.App",
+                        "Physics",
+                        fileName));
+                }
 
-        Assert.True(LocalForcePositionTransaction.Apply(
-            isForcePosition: false,
-            () => { calls++; return false; },
-            () => calls++,
-            () => calls++));
-        Assert.Equal(0, calls);
+                directory = directory.Parent;
+            }
+
+            throw new DirectoryNotFoundException("Could not find AcDream.slnx.");
+        }
     }
 }