fix(physics): #137 Task 3 — port obstruction_ethereal verbatim; retire AD-7 shim

Retail CPhysicsObj::FindObjCollisions (0x0050f050) only instant-skips when
BOTH ETHEREAL_PS (0x4) AND IGNORE_COLLISIONS_PS (0x10) are set (pc:276782).
ETHEREAL-alone sets sphere_path.obstruction_ethereal=1 (pc:276806) and
continues to the shape dispatch. BSPTREE::find_collisions (0x0053a496) routes
Path 1 (sphere_intersects_solid) when the flag is set (pc:323742): the open
door has no solid leaf at the doorway, so the test returns OK → player passes
through. CEnvCell::find_env_collisions (0x0052c144) clears the flag first so
ENV walls are never weakened (pc:309580, "D5 clear").

Changes:
- CollisionExemption.ShouldSkip: require BOTH bits for Gate-1 early-out
  (previously ETHEREAL alone returned true — the AD-7 shim). Divergence
  register row AD-7 deleted.
- SpherePath: add ObstructionEthereal field (mirrors retail
  SPHEREPATH.obstruction_ethereal).
- FindObjCollisionsInternal loop: set sp.ObstructionEthereal=(target&0x4)!=0
  before shape dispatch; clear it after (per-object clear pc:276989).
  Also clear at the null-BSP continue site to keep flag clean.
- FindEnvCollisions: clear sp.ObstructionEthereal=false at top (D5 clear
  pc:309580) — ENV cell walls are always solid.
- BSPQuery.FindCollisions Path 1: change `obj.Ethereal` (ObjectInfo.Ethereal,
  always false — dead code) to `path.ObstructionEthereal`. Gate now correctly
  mirrors retail pc:323742: PLACEMENT_INSERT || obstruction_ethereal.

Consume site change (BSPQuery.cs before/after):
  BEFORE: if (path.InsertType == InsertType.Placement || obj.Ethereal)
  AFTER:  if (path.InsertType == InsertType.Placement || path.ObstructionEthereal)
Mirrors retail pc:323742 exactly. obj.Ethereal was dead code (ObjectInfo.Ethereal
is never set true anywhere); the correct flag is SpherePath.ObstructionEthereal.

Tests: 1580 pass / 0 fail / 2 skip (was 1576/0/2 + 4 new in ObstructionEtherealTests.
CellarUp, CornerFlood, DoorCollision, HouseExitWalk all green — no wall regressions.

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

tests/AcDream.Core.Tests/Physics/CollisionExemptionTests.cs

@@ -42,16 +42,16 @@ public class CollisionExemptionTests
     }
 
     [Fact]
-    public void EtherealOnly_Skipped()
+    public void EtherealOnly_NotInstantSkipped()
     {
-        // L.2g slice 1b (2026-05-13): ETHEREAL alone exempts collision.
-        // Retail (acclient_2013_pseudo_c.txt:276782) required both bits,
-        // but ACE's Door.Open() broadcasts ETHEREAL alone — observed
-        // live: state=0x0001000C (HasPhysicsBSP | Ethereal | ReportCollisions).
-        // Pragmatic shortcut: widen the early-out to ETHEREAL alone so
-        // doors become passable when ACE flips the bit. Retail-server
-        // broadcasts (state=0x14+) still hit the same branch correctly.
-        Assert.True(CollisionExemption.ShouldSkip(
+        // Task 3 (2026-06-24): AD-7 shim retired. ETHEREAL alone (0x4) no
+        // longer instant-skips — retail requires BOTH ETHEREAL_PS (0x4) AND
+        // IGNORE_COLLISIONS_PS (0x10) for the Gate-1 early-out.
+        // ETHEREAL-alone takes the obstruction_ethereal path: ShouldSkip
+        // returns false → sp.ObstructionEthereal = true → BSP Path 1 fires
+        // (sphere_intersects_solid) → open door is passable because its BSP
+        // has no solid leaf at the opening. Retail pc:276782 / 0x0050f067.
+        Assert.False(CollisionExemption.ShouldSkip(
             targetState: ETHEREAL_PS,
             targetFlags: EntityCollisionFlags.None,
             moverState: ObjectInfoState.IsPlayer));