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>

src/AcDream.Runtime/Physics/RuntimeSetPositionState.cs

@@ -1318,6 +1318,16 @@ internal sealed class RuntimeSetPositionState : IDisposable
             allowDirectUnsealed: true);
     }
 
+    /// <summary>
+    /// C5a (2026-08-05): test seam. A pure pass-through to
+    /// <c>BeginAcceptedPlacementCore</c> (<c>captureMoverPreparationAuthority:
+    /// false</c>) kept ONLY so Runtime test fixtures can open a placement
+    /// operation directly; production reaches the core exclusively through
+    /// <see cref="Apply"/> and the authored sequence. The core it calls IS
+    /// the canonical path, so this wrapper is not a legacy path to delete —
+    /// deleting it would only buy broad mechanical test churn for zero
+    /// behaviour change (C5a scoping, disposition 3.7).
+    /// </summary>
     internal RuntimeEntityPlacementToken BeginAcceptedPlacement(
         RuntimeEntityRecord record,
         ulong expectedPositionAuthorityVersion,
@@ -1330,6 +1340,15 @@ internal sealed class RuntimeSetPositionState : IDisposable
             portal,
             captureMoverPreparationAuthority: false);
 
+    /// <summary>
+    /// C5a (2026-08-05): test seam. A pure pass-through to
+    /// <c>BeginAcceptedPlacementCore</c> (<c>captureMoverPreparationAuthority:
+    /// true</c>) kept ONLY so Runtime test fixtures can open an authored
+    /// placement operation directly; production reaches the core through the
+    /// authored sequence itself. See <see cref="BeginAcceptedPlacement"/> for
+    /// why this wrapper is a recorded deliberate exception, not a deletion
+    /// target.
+    /// </summary>
     internal RuntimeEntityPlacementToken BeginAuthoredPlacement(
         RuntimeEntityRecord record,
         ulong expectedPositionAuthorityVersion,