feat(physics): L.3a — wall-bounce velocity reflection on airborne hits

Three independent research agents converged: retail's "bouncy walls"
feel comes from CPhysicsObj::handle_all_collisions (acclient_2013_pseudo_c.txt:
282699-282715, ACE PhysicsObj.cs:2692-2697) which applies the canonical
reflection v_new = v - (1 + e) * dot(v, n) * n to the body's velocity
after every transition resolves. Player elasticity = 0.05 (5% bounce);
INELASTIC_PS = 0x20000 zeros velocity entirely (used by spell projectiles).

acdream had the data plumbed (PhysicsBody.Elasticity = 0.05 was already
set, ci.CollisionNormal was being populated in 8+ code paths) but
ResolveWithTransition discarded the normal before returning. Hence
"sticky walls on jumps" — perpendicular velocity got removed by
SlideSphere's geometric resolution, but never reflected back, so
hitting a wall mid-jump zeroed forward motion entirely instead of
producing a small push-back.

Files:
- PhysicsBody.cs: add PhysicsStateFlags.Inelastic = 0x20000.
- ResolveResult.cs: surface CollisionNormalValid + CollisionNormal.
- PhysicsEngine.cs:599-624: copy ci.CollisionNormal into ResolveResult
  before returning (both ok and partial paths).
- PlayerMovementController.cs:445-503: after position commit, apply
  reflection per the retail formula. Inelastic → zero velocity;
  else → reflect with v += n * -(dot(v,n) * (e + 1)).

apply_bounce rule (more conservative than retail by design):
- Sledding: retail's strict rule — bounce unless both grounded.
- Otherwise: bounce ONLY when both prev and now airborne. Suppress on
  landing (prev air, now ground) to avoid micro-bouncing on floor —
  the post-reflection upward Z defeats the controller's Velocity.Z<=0
  landing-snap gate. Retail's elasticity 0.05 makes the artifact
  visually imperceptible there; acdream's per-frame architecture
  amplifies it.

Tests: 1491 → 1491 still pass (existing AirborneFrames + WalkOffLedge
tests confirmed the conservative apply_bounce rule keeps landings
clean).

Live verification needed: jump into a wall mid-air — should produce a
visible bounce-back rather than sticking. Walking along corridor with
side-clip should still slide. Landing should still settle without
micro-bounce.

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

src/AcDream.Core/Physics/PhysicsBody.cs

@@ -31,6 +31,14 @@ public enum PhysicsStateFlags : uint
     ReportCollisions = 0x00000010,
     Gravity        = 0x00000400,  // bit 10 — apply downward gravity
     Hidden         = 0x00001000,
+    /// <summary>
+    /// L.3a (2026-04-30): retail INELASTIC_PS bit (acclient.h:2834).
+    /// When set, wall-collisions zero the velocity instead of reflecting.
+    /// Used by spell projectiles and missiles that should embed/explode on
+    /// impact rather than bounce. The player NEVER has this flag set —
+    /// player wall-hits use the reflection path with elasticity ~0.05.
+    /// </summary>
+    Inelastic      = 0x00020000,  // bit 17 — retail INELASTIC_PS
     Sledding       = 0x00800000,  // bit 23 — sledding (modified friction)
 }
 
@@ -44,6 +52,7 @@ public enum TransientStateFlags : uint
     None       = 0,
     Contact    = 0x00000001,  // bit 0 — touching any surface
     OnWalkable = 0x00000002,  // bit 1 — standing on a walkable surface
+    Sliding    = 0x00000004,  // bit 2 — carry sliding normal into next transition
     Active     = 0x00000080,  // bit 7 — object needs per-frame update
 }
 
@@ -87,6 +96,9 @@ public sealed class PhysicsBody
     /// <summary>Ground contact-plane normal (+0x130/134/138).</summary>
     public Vector3 GroundNormal { get; set; } = Vector3.UnitZ;
 
+    /// <summary>Last wall/object sliding normal (retail transient Sliding state).</summary>
+    public Vector3 SlidingNormal { get; set; }
+
     // ── persisted contact-plane state (retail PhysicsObj fields) ───────────
     //
     // Retail's PhysicsObj carries its last contact plane FORWARD across frames.
@@ -113,6 +125,18 @@ public sealed class PhysicsBody
     /// <summary>Whether the contact plane is a water surface (affects step behavior).</summary>
     public bool ContactPlaneIsWater { get; set; }
 
+    /// <summary>Whether the previous walkable polygon is available for edge slide.</summary>
+    public bool WalkablePolygonValid { get; set; }
+
+    /// <summary>Most recent walkable polygon plane (world-space).</summary>
+    public System.Numerics.Plane WalkablePlane { get; set; }
+
+    /// <summary>Most recent walkable polygon vertices (world-space).</summary>
+    public Vector3[]? WalkableVertices { get; set; }
+
+    /// <summary>Up vector used by the most recent walkable polygon probe.</summary>
+    public Vector3 WalkableUp { get; set; } = Vector3.UnitZ;
+
     /// <summary>Elasticity coefficient (+0xB0).</summary>
     public float Elasticity { get; set; } = 0.05f;