Fix: planar Y-lock in CharacterProcessor — no more climbing/floating on sloped colliders

Field report: walking onto hilly-looking colliders (the new convex rock
hulls are ramps) raised the character's Y, and with gravity zero +
SnapToGround off nothing ever brought it back down - the character
floated and its shadow drifted away.

Fix inside CharacterProcessor.PhysicsUpdate: capture the pre-update Y
and restore it (plus zero RelativeVelocity.y) after the CC sequence -
the character update itself can never change the movement plane, while
spawn/teleport writes made outside the update stay honored.
Deterministic + idempotent (rollback-safe), identical on server +
predicted client.

Verified live: player shoved into a cover-hull sloped edge decollided
~1 u horizontally with Y bit-exact at 1.0000 (the case that previously
rode up the slope); 466/466 EditMode, console clean.

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

Assets/_Project/Scripts/Simulation/Player/CharacterProcessor.cs

@@ -34,6 +34,7 @@ namespace ProjectM.Simulation
             ref CharacterComponent characterComponent = ref CharacterComponent.ValueRW;
             ref KinematicCharacterBody characterBody = ref CharacterDataAccess.CharacterBody.ValueRW;
             ref float3 characterPosition = ref CharacterDataAccess.LocalTransform.ValueRW.Position;
+            float planeY = characterPosition.y; // authored movement plane — restored after the update (see below)
 
             KinematicCharacterUtilities.Update_Initialize(
                 in this, ref context, ref baseContext,
@@ -100,6 +101,14 @@ namespace ProjectM.Simulation
             KinematicCharacterUtilities.Update_ProcessStatefulCharacterHits(
                 CharacterDataAccess.CharacterHitsBuffer,
                 CharacterDataAccess.StatefulHitsBuffer);
+
+            // Top-down planar lock: the world is one flat plane and gravity is zero, so any Y gained by
+            // step-handling/velocity-projection up a sloped collider (convex rock hulls etc.) would persist
+            // FOREVER — the character ends up floating and its shadow drifts away (07-09 field report).
+            // Restoring the pre-update Y forbids the CC update itself from ever changing the plane while
+            // spawn/teleport writes (made outside this update) stay honored. Deterministic + idempotent.
+            characterPosition.y = planeY;
+            characterBody.RelativeVelocity.y = 0f;
         }
 
         void HandleVelocityControl(ref CharacterUpdateContext context, ref KinematicCharacterUpdateContext baseContext)