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/PhysicsEngine.cs

@@ -1298,6 +1298,17 @@ public sealed class PhysicsEngine
                         body.ContactPlaneValid = false;
                     }
 
+                    // 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 — this is acdream's equivalent
+                    // per-resolve commit point. Mirrors whatever body.ContactPlaneIsWater was
+                    // just set to above (unchanged/stale in the no-valid-contact branch,
+                    // matching that branch's existing ContactPlaneIsWater behavior).
+                    if (body.ContactPlaneIsWater)
+                        body.TransientState |= TransientStateFlags.WaterContact;
+                    else
+                        body.TransientState &= ~TransientStateFlags.WaterContact;
+
                     // Publish frames_stationary_fall + carry it to the next frame via the Stationary*
                     // transient bits. Retail encodes these bits in handle_all_collisions (pc:282737-758);
                     // acdream co-locates the encode with the fsf writeback here (STRUCTURAL ADAPTATION,