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>

docs/architecture/retail-divergence-register.md

@@ -71,7 +71,6 @@ accepted-divergence entries (#96, #49, #50).
 | AD-4 | `point_in_cell` against an unhydrated CellBSP returns false (skip) rather than the null-node "inside" default; retail never queries unloaded cells | `src/AcDream.Core/Physics/CellTransit.cs:588` | The null-node default would make an unhydrated cell spuriously claim every point; skipping is the conservative streaming-safe choice | During hydration, a point genuinely inside a not-yet-loaded cell resolves outdoor/stale — transient membership misclassification driving wrong collision set and render root | `CEnvCell::find_visible_child_cell` :311397; cell-BSP vtable[0x84] |
 | AD-5 | Outdoor `point_in_cell` is an identity compare against the global XY-column cell from `LandDefs.AdjustToOutside` (no per-cell containment test) | `src/AcDream.Core/Physics/CellTransit.cs:865` | Landcells are disjoint 24 m columns — identity-compare against the column under the sphere centre is exactly equivalent to retail's per-candidate test | If block-origin/lcoord math is wrong at a landblock seam, the compare silently never matches — outdoor membership freezes at boundaries (the pre-#106 symptom) | `find_cell_list` pick pc:308788-308825; `CLandCell::point_in_cell` (get_block_offset pc:308804) |
 | AD-6 | Per-LANDBLOCK shadow re-flood on hydration vs retail per-CELL `recalc_cross_cells` | `src/AcDream.Core/Physics/ShadowObjectRegistry.cs:339` | The streaming unit IS the landblock; one hook per hydration event covers both race directions (entity-before-cells, cells-after-spawn) | Any cell-hydration path that doesn't raise the landblock hook leaves an entity's shadow set stale — walk-through / missing collisions in just-streamed cells | `CObjCell::init_objects` → `recalc_cross_cells`, 0x0052b420 / 0x00515a30 |
-| AD-7 | Full collision exemption on ETHEREAL alone; retail requires ETHEREAL_PS **and** IGNORE_COLLISIONS_PS (ETHEREAL-alone takes the unported `obstruction_ethereal` path) | `src/AcDream.Core/Physics/CollisionExemption.cs:78` | ACE's `Door.Open()` broadcasts ETHEREAL only (0x0001000C); without the shortcut, opened doors stay solid on ACE | ETHEREAL-only targets generate NO contact where retail records contact-but-allows-passage; against a retail-semantics server the bit means something different than we implement | pc:276782 (combined gate), :276795 (obstruction_ethereal) |
 | AD-8 | MoveTo arrival gate `max(minDistance, distanceToObject)`; retail tests `dist <= min_distance` only | `src/AcDream.Core/Physics/RemoteMoveToDriver.cs:161` | ACE ships the threshold in `distance_to_object` with `min_distance == 0`; without the max, monsters never "arrive" and oscillate at melee range (user-reported 2026-04-28) | A server using both wire fields with retail semantics + large `distance_to_object` makes remotes stop short of the retail arrival point | `MoveToManager::HandleMoveToPosition` chase-arrival |
 | AD-9 | 1.5 s stale-destination give-up timer on remote MoveTo (retail's MoveToManager runs until cancelled) | `src/AcDream.Core/Physics/RemoteMoveToDriver.cs:136` | Liveness guard sized to ACE's ~1 Hz re-emit cadence; prevents steering toward a stale destination after a missed cancel (the run-in-place symptom) | A server emitting MoveTo slower than ~1.5 s makes remotes freeze mid-chase and snap later instead of steering continuously | MoveToManager (no equivalent timeout) |
 | AD-10 | Remote slope projection relocated to the queue-empty/head-reached combiner boundary; retail projects inside `CTransition::adjust_offset` during the sweep | `src/AcDream.Core/Physics/PositionManager.cs:47` | Remote bodies don't run a full local transition sweep; boundary projection removes the ~5 Hz Z staircase on slopes, no-op on flat ground | The single-point terrain-normal sample can differ from the sweep's contact plane (cell boundaries, props underfoot) — remote Z drift / stair-stepping | `CTransition::adjust_offset` pc:272296-272346 |