feat(A): wire SweepEye to the verbatim update_viewer (start-cell + fallbacks)

Complete Render Residual A's faithful port: PhysicsCameraCollisionProbe.SweepEye
now mirrors SmartBox::update_viewer (acclient_2013_pseudo_c.txt:92761) end-to-end:

- Start cell (pc:92824-92844): indoor (>=0x100) seats the sweep at the head-PIVOT
  via PhysicsEngine.AdjustPosition (the cellar-lip case — feet in the low connector,
  head up at floor level); outdoor keeps the player cell.
- Sweep pivot -> sought-eye from the seated start cell (unchanged 0x5c viewer flags).
- Success (pc:92870): set_viewer(curr_pos), viewer_cell = curr_cell.
- Fallback 1 (pc:92878): AdjustPosition(sought_eye).
- Fallback 2 / no-cell (pc:92775, 92886): snap to player, viewer_cell = null. This
  also makes cellId==0 faithful (was returning the desired eye; retail snaps to
  player_pos) and adds the playerPos arg to ICameraCollisionProbe.SweepEye.

Supporting: ResolveResult.Ok surfaces FindTransitionalPosition's return (retail
find_valid_position != 0, pc:273898) so SweepEye knows when to fall back.

TDD: 11 new tests (FindVisibleChildCell 4, AdjustPosition 3, ResolveResult.Ok 2,
SweepEye orchestration 2). The seating test's RED proved the sweep does NOT auto-
advance feet->room, so the pivot-seated start cell is genuinely decisive. Core
1326 pass / 4 documented-fail / 1 skip; App 179 pass / 0 fail. No regression.

Per the live-capture finding, the visible payoff is the cellar-corner (point 3);
the cottage-room bluish void stays for residual C. Spec:
docs/superpowers/specs/2026-06-05-residual-a-camera-collision-design.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/ResolveResult.cs

@@ -27,4 +27,11 @@ public readonly record struct ResolveResult(
     bool CollisionNormalValid = false,
     /// <summary>Outward surface normal of the wall the sphere hit. Used
     /// by the velocity-reflection step. Pointing away from the wall.</summary>
-    Vector3 CollisionNormal = default);
+    Vector3 CollisionNormal = default,
+    /// <summary>Render Residual A — whether the underlying
+    /// <c>FindTransitionalPosition</c> found a valid position (retail
+    /// <c>find_valid_position != 0</c>, pc:273898). False when the sweep had no
+    /// start cell or was immediately stuck. The camera <c>SweepEye</c> reads this
+    /// to trigger <c>SmartBox::update_viewer</c>'s fallbacks. Default <c>true</c>
+    /// so existing callers are unaffected.</summary>
+    bool Ok = true);