fix(physics): water depth submersion + sphere-safety-push steep-slope correction

Two coupled physics fixes that together resolve "+Acdream walks on top of
water instead of submerged" and "brief Falling animation when running up
steep hills".

## 1. Water depth = physics adjustment, not rendering

Retail has NO separate water surface mesh. Characters visually submerge
in water because ValidateWalkable adds `waterDepth` to its signed-distance
check (ACE ObjectInfo.cs:124), letting the character's feet sit below the
terrain plane by that amount before the push-up fires. Rendered character
below rendered terrain = looks submerged.

Our ValidateWalkable didn't carry a waterDepth, so feet were always
snapped exactly to the plane. Water cells looked like walking on water.

Added:
 - TerrainSurface now carries per-vertex water flags (bits 2-6 of
   TerrainInfo → SurfChar lookup) and per-cell classification.
 - TerrainSurface.SampleWaterDepth(localX, localY) returns 0.0 (dry),
   0.45 (partial-water near water corner), 0.9 (entirely water). Deviates
   from retail's 0.1 fallback for "dry corner of partial-water cell" —
   that 0.1 destabilizes the "feet exactly on plane" contact-touch check
   in ValidateWalkable (dist > EPSILON, SetContactPlane skipped,
   ValidateTransition clears OnWalkable, gravity applies, character
   micro-falls each frame).
 - PhysicsEngine.SampleWaterDepth is the world-space wrapper.
 - FindEnvCollisions samples the per-point depth and forwards it.
 - ValidateWalkable adds +waterDepth to the signed-distance check (this
   is the ACE-line-124 port).

GameWindow.ApplyLoadedTerrain extracts the low byte of each TerrainInfo
ushort and passes it to the TerrainSurface ctor so classification works.

## 2. AdjustOffset safety-push threshold on sloped planes

The LocalSphere is positioned at `(0, 0, radius)` — center along world
+Z from the character root. On a tilted plane the sphere center's
perpendicular distance to that plane is `radius * Normal.Z`, NOT
`radius`. The original threshold `dist < radius - EPS` therefore fires
spuriously on every slope and the follow-up push-up lifts feet by
`radius * (sec θ - 1)` — 7 cm at 30°, 20 cm at 45°, 48 cm at 60°.

The steep-slope lift is large enough to break ValidateWalkable's
contact-touch check, ValidateTransition then clears OnWalkable,
calc_acceleration applies gravity, and the character flickers into the
Falling animation for ~0.3s while running uphill. User-observed on steep
hills after today's water-depth work made the artifact visible (before
that, general hover masked it).

Fix: the threshold is `radius * Normal.Z` (the natural resting distance
of a Z-axis sphere on the plane). The push fires only when feet are
actually penetrating below natural resting, not on any sloped plane.
ACE's Transition.cs AdjustOffset has the original threshold but the bug
is invisible server-side.

All 717 tests green. Water submersion + steep-slope running both
user-visually verified.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -110,6 +110,29 @@ public sealed class PhysicsEngine
         return null;
     }
 
+    /// <summary>
+    /// Sample the per-point water depth at the given world-space XY
+    /// (meters by which the character is allowed to sink below the
+    /// contact plane — 0.9 on fully-flooded water cells, 0.45 on
+    /// partial-water near a water corner, 0.1 on non-water corners of
+    /// partial-water cells, 0 on dry cells). Matches ACE
+    /// <c>ObjCell.get_water_depth</c>. Used by
+    /// <see cref="Transition"/> to visually submerge characters in water
+    /// without needing a separate water surface mesh.
+    /// </summary>
+    public float SampleWaterDepth(float worldX, float worldY)
+    {
+        foreach (var kvp in _landblocks)
+        {
+            var lb = kvp.Value;
+            float localX = worldX - lb.WorldOffsetX;
+            float localY = worldY - lb.WorldOffsetY;
+            if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
+                return lb.Terrain.SampleWaterDepth(localX, localY);
+        }
+        return 0f;
+    }
+
     /// <summary>
     /// Sample the outdoor terrain plane (Z + sloped normal) at the given
     /// world-space XY position. The returned <see cref="System.Numerics.Plane"/>