fix(physics): #265/#166 - stop zeroing grounded residual velocity, wire GroundNormal

Capture bisect (docs/research/2026-07-30-265-capture-bisect.md, mined
from artifacts/matrix-session2-resolve.jsonl records 3415-3434) traced
#265's lost roof slides / permanent landing freeze and #166's missing
downhill sled to a pre-existing (2026-07-20, ten days before Campaign P
- not a regression) mechanism in PlayerMovementController.cs's grounded
quantum block: it hand-zeroed Velocity.X/Y to exactly zero every tick
once OnWalkable whenever animation root motion drives the walk (the
production graphical local-player path), discarding any residual
horizontal momentum a fall left on the body before calc_friction
(AP-7/AD-55, already correctly ported) or PhysicsBody.
UpdatePhysicsInternal's Euler integrator ever got a chance to act on it.

Two changes:

1. PhysicsEngine.cs now syncs body.GroundNormal (the vector
   calc_friction dots velocity against, per retail
   CPhysicsObj::calc_friction 0x0050ee70's `contact_plane.Normal` read)
   from the committed ContactPlane.Normal at the same commit point that
   already publishes ContactPlane. GroundNormal had zero production
   writers before this and silently defaulted to Vector3.UnitZ forever
   - even surviving velocity would have been tested against a fake
   flat-ground normal on any real slope. Core-level, so player, remote,
   ordinary, and projectile movers all benefit uniformly.

2. PlayerMovementController.cs's grounded block no longer reconstructs
   Velocity at all for the animation-root-motion case (only the
   headless/test-controller get_state_velocity fallback still does,
   unchanged). Root motion continues to fully own commanded locomotion;
   this only stops destroying whatever Velocity already holds, letting
   it compose with root motion through the same ResolveWithTransition
   sweep exactly as retail's CPhysicsObj::UpdatePositionInternal
   composes both channels.

Symptom (a), the uphill-jump bounce, traces to a SEPARATE, byte-exact
(re-verified against acclient_2013_pseudo_c.txt:282647-282760),
already-closed retail mechanism (AD-25, PhysicsObjUpdate.
HandleAllCollisions's shouldReflect gate) - confirmed orthogonal to this
fix, not addressed here (see the research doc's as-fixed addendum §9.5).

Issue265SteepSlopeCaptureBisectTests.cs gains a composed harness
(ReplayRealRoofLandingComposed) mirroring PlayerMovementController.cs's
per-tick composition against Core types only, proving: the old model
reproduces the mined freeze exactly; the new model survives the landing
and slides continuously (the real captured geometry glides at constant
velocity per retail's own dot>=0.25 early-return - AP-7); a synthetic
dot<0.25 case shows genuine exponential decay via calc_friction; and a
synthetic uphill-bounce case proves the fix changes nothing about
HandleAllCollisions's reflection decision.

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

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -1300,6 +1300,18 @@ public sealed class PhysicsEngine
                         body.ContactPlane = ci.ContactPlane;
                         body.ContactPlaneCellId = ci.ContactPlaneCellId;
                         body.ContactPlaneIsWater = ci.ContactPlaneIsWater;
+                        // #265/#166 (2026-07-30): retail CPhysicsObj::calc_friction
+                        // (0x0050ee70) reads `this->contact_plane.Normal` directly off
+                        // the object (see PhysicsBody.calc_friction's doc comment).
+                        // acdream models that same field as the separate GroundNormal
+                        // property so isolated unit tests can drive calc_friction
+                        // without a full resolve, but nothing wrote it from a live
+                        // resolve before now -- calc_friction always saw the Vector3.UnitZ
+                        // default, i.e. every slope behaved like flat ground. Sync it
+                        // here, at the SAME commit point that already publishes
+                        // ContactPlane, so every caller (player, remote, ordinary,
+                        // projectile) gets a real slope normal for free.
+                        body.GroundNormal = ci.ContactPlane.Normal;
                     }
                     else if (ci.LastKnownContactPlaneValid)
                     {
@@ -1307,10 +1319,16 @@ public sealed class PhysicsEngine
                         body.ContactPlane = ci.LastKnownContactPlane;
                         body.ContactPlaneCellId = ci.LastKnownContactPlaneCellId;
                         body.ContactPlaneIsWater = ci.LastKnownContactPlaneIsWater;
+                        body.GroundNormal = ci.LastKnownContactPlane.Normal;
                     }
                     else
                     {
                         body.ContactPlaneValid = false;
+                        // GroundNormal left unchanged/stale -- matches ContactPlane's
+                        // own stale-retention pattern in this branch (comment above).
+                        // calc_friction only reads it while OnWalkable, and OnWalkable
+                        // cannot be true without a valid contact plane, so a stale
+                        // value here is never observed.
                     }
 
                     // AP-10 (Campaign P Slice P4, 2026-07-30): retail SetPositionInternal