docs(motion): #42 root cause confirmed — ResolveWithTransition airborne drift

A/B-tested 2026-05-05 with user observing retail-controlled remote:

  - With CellId fix removed: jumps render with geometrically-correct
    XY (no drift) but body falls through the floor.
  - With CellId fix applied: jumps land cleanly but arc shows ~1 m
    horizontal offset; snaps back on next UM.

Confirms the drift originates inside ResolveWithTransition, not from
wire data, local Euler error, or stale velocity. CellId fix kept in
place because falling through the floor is more disruptive than
~1 m visual jitter that resolves on next input.

#42 updated with the verified diagnosis, three ranked-by-probability
hypotheses for the in-sweep mechanism (initial-overlap depenetration
along non-+Z terrain normal is the leading candidate), three matching
fix paths, and a deterministic repro recipe for the next session.

The right next step is investigating PhysicsEngine.ResolveWithTransition
and comparing against retail's CTransition::find_valid_position
(docs/research/named-retail/) — out of scope for the L.3 motion port,
files as a follow-up PhysicsEngine bug.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -3434,12 +3434,10 @@ public sealed class GameWindow : IDisposable
 
                 // Adopt server's cell ID on every UP (airborne or grounded).
                 // Required by the legacy airborne path's per-tick
-                // ResolveWithTransition gate (rm.CellId != 0) — without this,
+                // ResolveWithTransition gate (rm.CellId != 0); without this
                 // an airborne player remote falls through the floor because
-                // the sphere sweep is skipped, K-fix15 landing detection never
-                // fires, and the body only re-grounds when the next UM forces
-                // ACE to broadcast a fresh IsGrounded=true UP that hits our
-                // landing transition branch below.
+                // the sphere sweep is skipped. Note: enabling the sweep also
+                // exposes a pre-existing depenetration bug — see #42.
                 rmState.CellId = p.LandblockId;
 
                 // Diagnostic (ACDREAM_REMOTE_VEL_DIAG=1): roll the previous