fix(physics): AP-10 - restore retail's 0.1m dry-corner water sink-in; wire WATER_CONTACT_TS

Campaign P Slice P4 item 2. TerrainSurface.SampleWaterDepth now returns 0.1
(was collapsed to 0) for a partially-water cell's dry corner, matching
retail's ObjCell.get_water_depth / calc_water_depth (via ACE's unambiguous
C# port). ValidateWalkable's formula was already byte-for-byte verbatim
(ACE ObjectInfo.ValidateWalkable line 124); only the constant was collapsed.

The old collapse's justification ("0.1 destabilizes the feet-exactly-on-plane
contact-touch check because dist > EPSILON skips SetContactPlane that tick")
is structurally true of retail too - traced and confirmed this slice: in ALL
THREE implementations (retail, ACE, acdream) a skipped touch-reassertion is
NOT a fall, because Contact/OnWalkable are STICKY -
PhysicsEngine.ResolveWithTransition's onGround computation ORs the fresh
per-call ContactPlaneValid with the seeded, persistent
PhysicsBody.TransientState.OnWalkable bit (itself written back by the
caller's own sticky TransientState). PhysicsEngine.SampleTerrainWalkable's
isWater = waterDepth >= 0.45f threshold means the restore does not flip the
dry corner's water classification (0.1 still < 0.45) - only the sink-in
depth changes. Full Core.Tests suite green (4038/2 skips, up from 4026)
proves the sticky-bit argument held in practice.

WATER_CONTACT_TS (TransientStateFlags.WaterContact, declared but never
written) is now mirrored alongside CONTACT_TS/ON_WALKABLE_TS at every commit
point that writes them: PhysicsObjUpdate.ApplySetPositionContact (projectiles
+ remote teleport), PhysicsObjUpdate.CommitSetPositionTransition (remote
teleport placement), and PhysicsEngine's per-resolve body-state commit (local
player + remote dead-reckoning + ordinary movers via ResolveWithTransition -
the actual SetPositionInternal-equivalent path). No signature changes needed:
body.ContactPlaneIsWater is already fresh by the time each function runs.

CollisionShadowVerifier audit: no change needed. It diffs graph-vs-flat BSP
traversal outcomes (ObjectInfo/CollisionInfo/SpherePath fields already
including ContactPlaneIsWater); it never touches PhysicsBody.TransientState,
and the water-depth constant is computed identically upstream of both
traversal modes, so it cannot introduce a new graph/flat divergence.

Filed #264 for the three items research explicitly left open (none block
this port): no confirmed retail consumer of WATER_CONTACT_TS was found (an
xref scan wasn't attempted - bitmask reads aren't text-greppable); the
CLandCell ENTIRELY_WATER ethereal/swim exemption from terrain collision was
not cross-checked; jump-in-water/swim-animation effects were not
investigated (out of physics/collision scope).

Conformance: Ap10WaterSemanticsTests covers SampleWaterDepth golden values
(NotWater/EntirelyWater/PartiallyWater wet+dry corners), the isWater
threshold non-flip, WaterContact mirroring in both PhysicsObjUpdate
functions, and two settle-to-rest end-to-end PhysicsEngine.ResolveWithTransition
scenarios (water: sinks exactly waterDepth below the plane and sets
WaterContact; dry: rests exactly on the plane and clears any stale
WaterContact bit).

Register: retired AP-10 (92 active AP rows, down from 93).

AcDream.Core.Tests: 4038 passed, 2 skipped, 0 failed.

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

src/AcDream.Core/Physics/PhysicsObjUpdate.cs

@@ -42,6 +42,16 @@ public static class PhysicsObjUpdate
         else
             body.TransientState &= ~TransientStateFlags.OnWalkable;
 
+        // AP-10 (Campaign P Slice P4, 2026-07-30): retail SetPositionInternal
+        // (0x005153e5-0051545f) writes WATER_CONTACT_TS in the same statement
+        // block as CONTACT_TS, immediately after. body.ContactPlaneIsWater is
+        // already current by the time this runs (every caller sets it, or it
+        // carries over from the prior tick, before calling here).
+        if (body.ContactPlaneIsWater)
+            body.TransientState |= TransientStateFlags.WaterContact;
+        else
+            body.TransientState &= ~TransientStateFlags.WaterContact;
+
         body.calc_acceleration();
     }
 
@@ -96,6 +106,15 @@ public static class PhysicsObjUpdate
         else
             body.TransientState &= ~TransientStateFlags.OnWalkable;
 
+        // AP-10 (Campaign P Slice P4, 2026-07-30): mirror WATER_CONTACT_TS
+        // alongside CONTACT_TS/ON_WALKABLE_TS, same as ApplySetPositionContact.
+        // Callers (e.g. RemoteTeleportPlacement) already set body.ContactPlaneIsWater
+        // before invoking this commit.
+        if (body.ContactPlaneIsWater)
+            body.TransientState |= TransientStateFlags.WaterContact;
+        else
+            body.TransientState &= ~TransientStateFlags.WaterContact;
+
         if (!previousOnWalkable && finalOnWalkable)
         {
             hitGround?.Invoke();