feat(R1-P4): verbatim update_internal / update / advance_to_next_animation

The CSequence frame-advance core (gaps G3/G4/G5/G6/G8/G9/G19),
ACE-verified skeleton + retail constants + the P0-pinned leftover carry:

- update_internal (0x005255d0): frame_number += framerate*dt; overshoot
  clamps to the RAW high/low frame with leftover-time computation +
  animDone; the pose/physics/hook triple fires for EVERY crossed integer
  frame (ascending fwd / descending rev, strict > < boundaries, NO
  epsilon, NO safety cap); AnimDone is a LIST-STRUCTURE gate (head !=
  first_cyclic) queuing the global AnimDoneHook; iterative loop carries
  the leftover into the next node (P0 pin — a lag spike fast-forwards
  through multiple queued nodes in one tick).
- advance_to_next_animation (0x005252b0): four pose ops per transition
  (subtract1 outgoing @ current frame + residual physics; step — fwd
  wraps to first_cyclic, REVERSE wraps to the LIST TAIL, asymmetric by
  design; reseed from the incoming direction-aware boundary; combine
  incoming + physics). Pose ops run in BOTH directions — ACE's
  framerate-sign gates are ACE-isms, decomp is unconditional modulo the
  degenerate-framerate guard.
- update (0x00525b80): non-empty -> update_internal + apricot; empty +
  frame -> accumulated-physics free motion (G8).
- execute_hooks (0x00524830): direction filter (Both or match) QUEUING
  into the IAnimHookQueue host seam (stands in for CPhysicsObj.anim_hooks
  + the AnimDoneHook singleton; drain placement moves in R6). Null part
  frame guarded (documented safe divergence vs retail's latent deref).
- FrameOps.Combine/Subtract1: the AFrame pose composition (verified
  math from the pre-R1 port, now against DatReaderWriter Frame).

10 conformance tests: single-tick goldens, exact-integer boundary
sit (the old #61 flash class), cyclic wrap without AnimDone,
link->cycle fast-forward proving hook order (link hooks -> ANIMDONE ->
cycle hooks) AND the carry, reverse descending hooks with the OOB
high+1 start, reverse tail-wrap, zero-framerate physics-only,
empty-list free motion, pos-frame root motion into the caller Frame,
apricot trim via update. Full suite 3346 green.

R1 remaining: P5 (adapter cutover — AnimationSequencer rehosted on the
core, legacy epsilon/stale-head/safety-cap DELETED) + P6 (root-motion
wiring + API narrowing).

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

src/AcDream.Core/Physics/Motion/FrameOps.cs

@@ -55,4 +55,28 @@ public static class FrameOps
     /// global through the orientation, then <see cref="GRotate"/>.</summary>
     public static void Rotate(Frame frame, Vector3 rotationLocal)
         => GRotate(frame, Vector3.Transform(rotationLocal, frame.Orientation));
+
+    /// <summary>
+    /// <c>Frame::combine</c> as used by the CSequence pose path (ACE
+    /// <c>AFrame.Combine</c>, verified against the pre-R1 acdream port):
+    /// apply the pose — origin += rotate(pos.Origin by orientation), then
+    /// orientation ∘= pos.Orientation.
+    /// </summary>
+    public static void Combine(Frame frame, Frame pos)
+    {
+        frame.Origin += Vector3.Transform(pos.Origin, frame.Orientation);
+        frame.Orientation = Quaternion.Normalize(frame.Orientation * pos.Orientation);
+    }
+
+    /// <summary>
+    /// <c>Frame::subtract1</c> — un-apply the pose: orientation ∘=
+    /// conj(pos.Orientation) FIRST, then origin −= rotate(pos.Origin by the
+    /// UPDATED orientation) (inverse order of <see cref="Combine"/>).
+    /// </summary>
+    public static void Subtract1(Frame frame, Frame pos)
+    {
+        frame.Orientation = Quaternion.Normalize(
+            frame.Orientation * Quaternion.Conjugate(pos.Orientation));
+        frame.Origin -= Vector3.Transform(pos.Origin, frame.Orientation);
+    }
 }