fix(anim): instant-engage Falling cycle on jump start (skip transition link)

User reported transition from running to jumping looked
slow -- the character stood still for ~100 ms at the start
of the jump before the legs folded into Falling.

Root cause: AnimationSequencer.SetCycle resolves a
transition link (e.g. RunForward -> Falling) from the
motion table and enqueues those non-looping link frames
BEFORE the Falling cycle. The link is the "stop running,
prepare to fall" anim -- a few frames of standing-style
pose. While it drained, the character looked frozen.

Fix: SetCycle gains a skipTransitionLink parameter. When
true, the GetLink call is bypassed AND the entire queue is
cleared (so any in-flight non-cyclic frames from a
previous transition don't continue draining). Only the
target cycle gets enqueued, cursor goes straight to its
start.

Both call sites pass true for Falling:
  - OnLiveVectorUpdated (remote-jump VectorUpdate handler)
  - UpdatePlayerAnimation (local airborne path) when
    animCommand == Falling. Other transitions
    (Walk -> Run, Run -> Ready, etc.) keep the link --
    smooth transitions stay smooth, only the jump start
    is hard-cut.

Tests stay 1222 green.

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

src/AcDream.Core/Physics/AnimationSequencer.cs

@@ -322,7 +322,15 @@ public sealed class AnimationSequencer
     /// <param name="style">MotionCommand style / stance (e.g. NonCombat 0x003D0000).</param>
     /// <param name="motion">Target motion command (e.g. WalkForward 0x45000005).</param>
     /// <param name="speedMod">Speed multiplier applied to framerates (1.0 = normal).</param>
-    public void SetCycle(uint style, uint motion, float speedMod = 1f)
+    /// <param name="skipTransitionLink">K-fix18 (2026-04-26): when true, do
+    /// NOT enqueue the transition-link frames between the previous and
+    /// new cycle. Used when the caller wants the new cycle to engage
+    /// instantly — e.g. swapping to Falling on a jump start, where the
+    /// RunForward→Falling link is a short "stop running" pose that
+    /// makes the jump look delayed (legs stand still for ~100 ms while
+    /// the link drains, then fold into Falling). Defaults to false to
+    /// preserve normal smooth transitions for everything else.</param>
+    public void SetCycle(uint style, uint motion, float speedMod = 1f, bool skipTransitionLink = false)
     {
         // ── adjust_motion: remap left→right / backward→forward variants ───
         // ACE MotionInterp.cs:394-428.  The MotionTable never stores TurnLeft,
@@ -372,9 +380,12 @@ public sealed class AnimationSequencer
         // CurrentSpeedMod defaults to 1.0 (positive) on a fresh sequencer,
         // so a Ready → WalkBackward transition correctly enters GetLink's
         // negative-speed (reversed-key) branch.
-        MotionData? linkData = CurrentMotion != 0
-            ? GetLink(style, CurrentMotion, CurrentSpeedMod, adjustedMotion, adjustedSpeed)
-            : null;
+        // K-fix18: when the caller asked to skip the transition link
+        // (instant-engage cases like Falling on jump start), force
+        // linkData to null so only the cycle gets enqueued.
+        MotionData? linkData = (skipTransitionLink || CurrentMotion == 0)
+            ? null
+            : GetLink(style, CurrentMotion, CurrentSpeedMod, adjustedMotion, adjustedSpeed);
 
         // Resolve target cycle using the ADJUSTED motion (TurnRight not TurnLeft).
         int cycleKey = (int)(((style & 0xFFFFu) << 16) | (adjustedMotion & 0xFFFFFFu));
@@ -384,6 +395,19 @@ public sealed class AnimationSequencer
         // been played yet (ACE behaviour: non-cyclic anims drain naturally).
         ClearCyclicTail();
 
+        // K-fix18: when the caller asked for instant-engage, ALSO drain
+        // any in-flight non-cyclic transition frames from the previous
+        // cycle. Without this, the old RunForward → ??? link would
+        // continue draining for ~100 ms before the new Falling cycle
+        // starts, defeating the "skip the link" intent.
+        if (skipTransitionLink)
+        {
+            _queue.Clear();
+            _currNode = null;
+            _firstCyclic = null;
+            _framePosition = 0.0;
+        }
+
         // Clear sequence-wide physics before the rebuild. Retail's
         // GetObjectSequence calls sequence.clear_physics() before each
         // add_motion chain (MotionTable.cs L100-L101, L152-L153).