feat(anim): motion-action-queue infrastructure + retail jump-is-physics-only note

Adds AnimationSequencer.PlayAction as the proper path for Action and
Modifier-class motions (the MotionTable.Modifiers dict, distinct from
Cycles). Action nodes are inserted before the looping cyclic tail so
they drain once and the cycle resumes naturally — leveraging the
sequencer's existing "non-looping head drains, cyclic tail wraps"
queue semantics.

What this does:
- New AnimationSequencer.PlayAction(motionCommand, speedMod=1f):
  - Resolves (style<<16) | (motion&0xFFFFFF) from MotionTable.Modifiers
  - Falls back to (motion&0xFFFFFF) plain key
  - Silent no-op when not found (some motion tables lack these)
  - Inserts AnimNodes before _firstCyclic; re-points the cursor when on
    the cyclic tail so the action plays immediately
- New MotionCommand.Jump (0x2500003B) + MotionCommand.FallDown (0x10000050)
  constants.
- GameWindow.UpdatePlayerAnimation fires PlayAction(Jump) on
  result.JumpExtent.HasValue and PlayAction(FallDown) on JustLanded.

Key research finding: retail does NOT animate jumps.
- ACE Player.HandleActionJump explicitly clears PendingMotions and sets
  IsAnimating=false during a jump (Player.cs:914-915).
- Empirical verification: the player humanoid's MotionTable only has 8
  Modifier entries — all TurnRight/SideStepRight stance variants. No
  Jump (0x2500003B) or FallDown (0x10000050) entries.
- Jump is a physics-only action: the character keeps whatever cycle
  was active (walk/run/idle) while the physics body arcs through the
  air. There is no "raise arms to jump" pose in retail.

PlayAction is still called on jump/land as a safety hatch for creature
Setups that DO carry leap animations in their Modifiers dict (drudge
jumps, monster pounces, etc.). For player humanoids it's a no-op. The
infrastructure is also ready for future emote/combat actions that
legitimately use the Modifiers dict.

470 tests pass, build clean.

src/AcDream.Core/Physics/MotionInterpreter.cs

@@ -47,6 +47,17 @@ public static class MotionCommand
     public const uint SideStepLeft      = 0x65000010u;
     /// <summary>0x40000008 — Fallen (lying on ground).</summary>
     public const uint Fallen            = 0x40000008u;
+    /// <summary>
+    /// 0x2500003B — Jump (Modifier action; played via
+    /// <see cref="AnimationSequencer.PlayAction"/>). NOT a SubState — it
+    /// overlays the current cycle via the motion table's Modifiers dict.
+    /// </summary>
+    public const uint Jump              = 0x2500003Bu;
+    /// <summary>
+    /// 0x10000050 — FallDown (Action; the landing animation played after
+    /// a jump. Enqueued via <see cref="AnimationSequencer.PlayAction"/>).
+    /// </summary>
+    public const uint FallDown          = 0x10000050u;
     /// <summary>0x10000057 — Dead.</summary>
     public const uint Dead              = 0x10000057u;
     /// <summary>0x41000011 — Crouch lower bound for blocked-jump check.</summary>