refactor(physics): delete legacy PhysicsEngine.Resolve/ResolvePlacement/HasCellSurface (C5a, AP-1/AD-1)

Member-wise deletion of the three legacy resolver members named in
docs/research/2026-08-05-c5a-contract.md: PhysicsEngine.Resolve,
PhysicsEngine.HasCellSurface, and PhysicsEngine.ResolvePlacement. An
exhaustive receiver census over src/ found zero production callers of any
of the three — every production placement writer already reaches the
canonical PhysicsEngine.SetPosition transaction exclusively through
RuntimeSetPositionState (three call sites total). The deletion is purely
member-wise: IsSpawnCellReady and AdjustPosition, which shared the same
source region as the deleted members, are preserved byte-identical — every
remaining production caller of either (including PhysicsCameraCollisionProbe,
AdjustPosition's sole surviving production caller) is unaffected.

Companion changes:
- PlayerMovementController's 3-argument SetPosition test overload is renamed
  to SeedPlacementForTest (internal) and CommitPreparedPosition is deleted;
  83 call sites across 19 test files were mechanically renamed to match.
- Seven pinned test dispositions from the contract are executed:
  3.1 (PhysicsEngineTests.cs: 11 legacy-resolver tests deleted, 6
  ResolveWithTransition tests kept), 3.2/3.3/3.4 (re-point to canonical
  SetPosition, with TransitionScratchDifferentialTests.cs additionally
  gaining positive IsCommitted assertions after each bitwise comparison so
  the differential proves a placement actually committed, not just that two
  possibly-uncommitted results match), 3.5 (Runtime rename), and 3.6
  (PlayerMovementPlacementTransactionTests.cs rewritten — its xmldoc now
  states plainly that the render-root publish moved to
  RuntimeSetPositionState.cs, but the sticky-release relocation claim was
  false and is retracted; this disposition's coverage loss is the sticky
  release path, not silently absorbed elsewhere).
- Stale `PhysicsEngine.Resolve`/`Resolve` doc citations in CellTransit.cs,
  PlayerMovementController.cs, and HeadlessSessionWorldProjection.cs are
  corrected to name the surviving canonical entry points by symbol
  (SetPosition, AdjustSetPosition/AdjustPosition, ResolveWithTransition)
  rather than fragile line numbers.

Retires AP-1 and AD-1 in docs/architecture/retail-divergence-register.md:
both rows described production zero-delta placement routing remaining on
the legacy resolver pending the Slice 4B2/4B route cutover; that resolver
no longer exists, so the condition each row tracked is now structurally
false rather than merely narrowed. AP-145 (routed through the prior commit)
and this commit's AP-1/AD-1 together bring the section counts to 101 AP / 47
AD active rows.

Builds on the AP-145 fix (previous commit) — this commit's staged tree was
independently rebuilt and its four suites independently rerun on top of
that commit before this commit was created, in addition to the combined
rebuild/rerun below.

Full-solution build: 0 errors (21 pre-existing warnings, all unrelated).
Suite results (combined tree): Core 4270/4271 passed (1 skip; the single
DatSoundCacheTests concurrent-decode-dedup failure is a known load-sensitive
race, confirmed passing standalone and unrelated to this change), Runtime
1176/1176, Headless 86/86, App 4132/4135 (3 skips).

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

tests/AcDream.Headless.Tests/HeadlessSessionHostTests.cs

@@ -402,9 +402,9 @@ public sealed class HeadlessSessionHostTests
         PlayerMovementController controller =
             Assert.IsType<PlayerMovementController>(
                 runtime.MovementOwner.Controller);
-        controller.SetPosition(
+        controller.SeedPlacementForTest(
             new Vector3(48f, 49f, 50f),
-            0xA9B40001u);
+            0xA9B40001u, new Vector3(48f, 49f, 50f));
         projection.ProjectPosition(
             record,
             isLocalPlayer: true,
@@ -533,9 +533,9 @@ public sealed class HeadlessSessionHostTests
         PlayerMovementController controller =
             Assert.IsType<PlayerMovementController>(
                 runtime.MovementOwner.Controller);
-        controller.SetPosition(
+        controller.SeedPlacementForTest(
             new Vector3(48f, 49f, 50f),
-            0xA9B40001u);
+            0xA9B40001u, new Vector3(48f, 49f, 50f));
         projection.ProjectPosition(
             record,
             isLocalPlayer: true,
@@ -727,9 +727,9 @@ public sealed class HeadlessSessionHostTests
         PlayerMovementController controller =
             Assert.IsType<PlayerMovementController>(
                 runtime.MovementOwner.Controller);
-        controller.SetPosition(
+        controller.SeedPlacementForTest(
             new Vector3(48f, 49f, 50f),
-            0xA9B40001u);
+            0xA9B40001u, new Vector3(48f, 49f, 50f));
         projection.ProjectPosition(
             record,
             isLocalPlayer: true,
@@ -878,9 +878,9 @@ public sealed class HeadlessSessionHostTests
             Assert.IsType<PlayerMovementController>(
                 runtime.MovementOwner.Controller);
 
-        controller.SetPosition(
+        controller.SeedPlacementForTest(
             new Vector3(48f, 49f, 50f),
-            0xA9B40001u);
+            0xA9B40001u, new Vector3(48f, 49f, 50f));
         projection.ProjectPosition(
             record,
             isLocalPlayer: true,
@@ -1583,7 +1583,7 @@ public sealed class HeadlessSessionHostTests
             record.Snapshot,
             replaceGeneration: false));
         var controller = new PlayerMovementController(engine);
-        controller.SetPosition(
+        controller.SeedPlacementForTest(
             new Vector3(96f, 97f, 50f),
             0xA9B40001u,
             new Vector3(96f, 97f, 50f));