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/ResolveResult.cs

@@ -6,8 +6,25 @@ namespace AcDream.Core.Physics;
 /// Result of <see cref="PhysicsEngine.Resolve"/>: the validated
 /// position after collision, the cell the entity ended up in,
 /// and whether they're standing on a surface.
+///
+/// <para>
+/// L.3a (2026-04-30): added optional collision-normal fields so the
+/// caller (typically <see cref="AcDream.App.Input.PlayerMovementController"/>)
+/// can apply retail's velocity-reflection bounce
+/// (<c>v_new = v - (1 + elasticity) * dot(v, n) * n</c>) to the
+/// PhysicsBody after the geometric resolve completes. ACE port mirror:
+/// <c>references/ACE/Source/ACE.Server/Physics/PhysicsObj.cs:2692-2697</c>;
+/// retail equivalent: <c>CPhysicsObj::handle_all_collisions</c> at
+/// <c>acclient_2013_pseudo_c.txt:282699-282715</c>.
+/// </para>
 /// </summary>
 public readonly record struct ResolveResult(
     Vector3 Position,
     uint CellId,
-    bool IsOnGround);
+    bool IsOnGround,
+    /// <summary>True when a wall collision occurred during this resolve
+    /// and <see cref="CollisionNormal"/> is meaningful.</summary>
+    bool CollisionNormalValid = false,
+    /// <summary>Outward surface normal of the wall the sphere hit. Used
+    /// by the velocity-reflection step. Pointing away from the wall.</summary>
+    Vector3 CollisionNormal = default);