feat: LTX-2 legal-frame helpers (core/ltx2.py)

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

core/ltx2.py

@@ -0,0 +1,26 @@
+"""LTX-2 frame-count math. Legal F satisfy F % 8 == 1 (8x temporal + 1)."""
+
+
+def is_legal_frames(f: int) -> bool:
+    return f >= 9 and f % 8 == 1
+
+
+def legal_frames(min_f: int = 9, max_f: int = 1000) -> list[int]:
+    start = max(9, min_f + ((1 - min_f) % 8))   # first 8k+1 >= min_f
+    return list(range(start, max_f + 1, 8))
+
+
+def nearest_legal_frames(f: int) -> int:
+    if f <= 9:
+        return 9
+    low = ((f - 1) // 8) * 8 + 1
+    high = low + 8
+    return low if (f - low) <= (high - f) else high
+
+
+def duration_for_frames(frames: int, fps: float) -> float:
+    return frames / fps
+
+
+def frames_for_duration(duration: float, fps: float) -> int:
+    return nearest_legal_frames(round(duration * fps))