feat(R1-P3): verbatim apply_physics + Frame rotate/grotate

- CSequence.ApplyPhysics (0x00524ab0): copysign semantics — magnitude
  from the quantum arg, sign from the sign-source arg (call sites pass
  1/framerate + signed elapsed); origin += velocity*signed, then
  rotate(omega*signed).
- FrameOps.GRotate (0x005357a0): axis-angle quaternion (angle=|v|,
  half-angle sin/cos) PREMULTIPLIED onto the orientation — incremental
  rotation in WORLD space; |v|^2 < F_EPSILON^2 skipped.
- FrameOps.Rotate (0x004525b0): local rotation vector mapped through
  the frame's local->global rotation, then GRotate.

7 numeric conformance tests (copysign matrix, global-space composition,
local->global mapping equivalence, epsilon gate).

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

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

@@ -253,6 +253,26 @@ public sealed class CSequence
     /// <summary><c>get_curr_frame_number</c> (0x005249d0).</summary>
     public int GetCurrFrameNumber() => (int)Math.Floor(FrameNumber);
 
+    // ── apply_physics (0x00524ab0, §19) ─────────────────────────────────
+
+    /// <summary>
+    /// Accumulated-physics root motion: advance <paramref name="frame"/> by
+    /// <see cref="Velocity"/>/<see cref="Omega"/> over a signed quantum —
+    /// MAGNITUDE from <paramref name="quantum"/>, SIGN from
+    /// <paramref name="signSource"/> (retail copysign semantics; call sites
+    /// pass 1/framerate as magnitude and the signed elapsed time as sign).
+    /// </summary>
+    public void ApplyPhysics(Frame frame, double quantum, double signSource)
+    {
+        double signed = Math.Abs(quantum);
+        if (signSource < 0.0)
+            signed = -signed;
+
+        float sq = (float)signed;
+        frame.Origin += Velocity * sq;
+        FrameOps.Rotate(frame, Omega * sq);
+    }
+
     // ── internal cursors for P4 (update_internal operates on nodes) ─────
 
     internal LinkedListNode<AnimSequenceNode>? CurrAnimNode