test(R2-Q4a): pre-cutover adapter trace goldens

11 scripted scenarios (spawn idle, idle→walk link, walk→run cyclic-to-
cyclic, fast re-speed, backward-walk remap, stop-settle fallback, emote
action, K-fix18 skip-link, turn modifier, style change, link-drain
sentinel count) snapshotting the CSequence core's list state after every
SetCycle/PlayAction against the LEGACY adapter (fast-path + Fix B +
stop-anim fallback + G17 gate). These are the parity bar for the Q4
PerformMovement cutover — intentional post-cutover changes get
EXPECTED-DIFF annotations, everything else must stay byte-identical.

Notable pre-cutover behavior captured: the S6 link-before-link stacking
(old reversed link kept mid-drain while the settle link appends behind).

Adds the internal AnimationSequencer.Core test seam.

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

src/AcDream.Core/Physics/AnimationSequencer.cs

@@ -159,6 +159,9 @@ public sealed class AnimationSequencer
     public int QueueCount => _core.Count;
     public bool HasCurrentNode => _core.CurrAnim != null;
 
+    /// <summary>Test seam (Q4 trace conformance): the verbatim CSequence core.</summary>
+    internal CSequence Core => _core;
+
     /// <summary>
     /// Diagnostic snapshot of the core's <c>curr_anim</c> identity + frame
     /// state, for the per-tick CURRNODE log line in