fix(physics): jump apex velocity zeroing bug

SmallVelocity threshold (0.25 m/s) in UpdatePhysicsInternal was zeroing
velocity every frame while airborne at the jump apex. With vel~0.01 m/s
and gravity adding only 0.012/frame, the zeroing won every frame and
the character got stuck at peak height forever.

Fix: only apply small-velocity zeroing when OnWalkable (grounded).
While airborne, gravity must accumulate freely through the zero-crossing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -97,6 +97,8 @@ public sealed class PlayerMovementController
 
     // Jump charge state.
     private bool _jumpCharging;
+    private int _debugSpeedLogCounter;
+    private int _debugJumpFrames;
     private float _jumpExtent;
     private const float JumpChargeRate = 1.0f; // 0→1 over 1 second
 
@@ -244,6 +246,8 @@ public sealed class PlayerMovementController
             else if (input.StrafeLeft)
                 localX = -MotionInterpreter.SidestepAnimSpeed * 0.5f;
 
+            if (input.Forward && _debugSpeedLogCounter++ % 60 == 0)
+                Console.WriteLine($"DEBUG speed: stateVel.Y={stateVel.Y:F2} MyRunRate={_motion.MyRunRate:F3} localY={localY:F2}");
             _body.set_local_velocity(new Vector3(localX, localY, savedWorldVz));
         }
 
@@ -271,6 +275,12 @@ public sealed class PlayerMovementController
             {
                 _motion.LeaveGround();
                 outJumpExtent = _jumpExtent;
+                _debugJumpFrames = 500; // log until landing
+                Console.WriteLine($"DEBUG jump FIRED: extent={_jumpExtent:F2} vel={_body.Velocity} onWalk={_body.OnWalkable}");
+            }
+            else
+            {
+                Console.WriteLine($"DEBUG jump FAILED: extent={_jumpExtent:F2} result={jumpResult} onWalk={_body.OnWalkable} contact={_body.InContact}");
             }
             _jumpCharging = false;
             _jumpExtent   = 0f;
@@ -285,6 +295,12 @@ public sealed class PlayerMovementController
         _body.calc_acceleration();
         _body.UpdatePhysicsInternal(dt);
 
+        if (_debugJumpFrames > 0)
+        {
+            _debugJumpFrames--;
+            Console.WriteLine($"DEBUG jump frame: pos.Z={_body.Position.Z:F3} vel.Z={_body.Velocity.Z:F3} onWalk={_body.OnWalkable} accel.Z={_body.Acceleration.Z:F1} dt={dt:F4}");
+        }
+
         // ── 5. Terrain/cell Z snap and ground-contact detection ───────────────
         // Use PhysicsEngine.Resolve to find the ground surface Z under the player.
         // We pass a zero delta because PhysicsBody already moved the position.
@@ -310,7 +326,11 @@ public sealed class PlayerMovementController
                     _body.Velocity = new Vector3(_body.Velocity.X, _body.Velocity.Y, 0f);
 
                 if (wasAirborne)
+                {
+                    Console.WriteLine($"DEBUG LANDED: bodyZ={bodyZ:F3} groundZ={groundZ:F3} vel.Z={_body.Velocity.Z:F3}");
                     _motion.HitGround();
+                    _debugJumpFrames = 0;
+                }
             }
             else
             {

src/AcDream.App/Rendering/GameWindow.cs

@@ -960,21 +960,22 @@ public sealed class GameWindow : IDisposable
             && newCycle.HighFrame > newCycle.LowFrame
             && newCycle.Animation.PartFrames.Count > 1;
 
-        if (!newCycleIsGood)
+        // Wire server-echoed RunRate BEFORE the animation early-return.
-            return;
+        // If the cycle can't resolve (bad stance), we still need the speed.
-
-        // Wire server-echoed RunRate into the player's MotionInterpreter.
-        // The server broadcasts the character's real Run-skill-derived ForwardSpeed
-        // in UpdateMotion; without this the player would always move at 4.0 m/s
-        // (ForwardSpeed = 1.0 hardcoded in MotionInterpreter defaults).
         if (_playerController is not null
             && update.Guid == _playerServerGuid
             && update.MotionState.ForwardSpeed.HasValue
             && update.MotionState.ForwardSpeed.Value > 0f)
         {
+            Console.WriteLine($"DEBUG RunRate: guid={update.Guid:X8} fwd={update.MotionState.ForwardSpeed.Value:F3}");
             _playerController.ApplyServerRunRate(update.MotionState.ForwardSpeed.Value);
         }
 
+        if (!newCycleIsGood)
+            return;
+
+        // (RunRate wiring moved above the early-return)
+
         // Sequencer path
         if (ae.Sequencer is not null)
         {

src/AcDream.Core/Physics/PhysicsBody.cs

@@ -298,7 +298,10 @@ public sealed class PhysicsBody
             calc_friction(dt, velocityMag2);
 
             // If velocity fell below the "small" threshold after friction, stop.
-            if (velocityMag2 - SmallVelocitySquared < 0.0002f)
+            // Only apply when grounded — while airborne, gravity must accumulate
+            // even when velocity is near zero (e.g., at jump apex).
+            if (velocityMag2 - SmallVelocitySquared < 0.0002f
+                && TransientState.HasFlag(TransientStateFlags.OnWalkable))
                 Velocity = Vector3.Zero;
 
             // Euler integration: position += v*dt + 0.5*a*dt²