Chunking v2: keep clips that span a chunk boundary (overlap + seek)

v1 dropped any overlay video or audio clip whose start fell outside a chunk's
window, so a multi-slide background video — or looping background music — silently
vanished from every chunk after the one it started in, diverging from a full render.

v2 includes any clip that OVERLAPS the window and seeks into clips that began
earlier so they resume mid-clip at the seam instead of restarting:
- _extract_video_events: overlap test + VideoEvent.skip_override (loop-aware phase);
  the input -ss also seeks the clip's embedded audio (tvaud).
- _extract_audio_events: overlap test + AudioEvent.src_offset (loop phase / linear);
  renderer honors it in the loop-with-pauses, standard-loop, and one-shot paths.
- _chunk_boundary_span_warnings downgraded from "will be dropped" to informational.

Full (non-chunked) render is unchanged: time_range=None leaves skip_override/
src_offset at defaults. Slides already used overlap; outros are last-chunk-only.

Plan-level checks in tests/test_chunking_v2.py (all green). Remaining gate: render-
seam frame/phase validation on the rig; crossfade-loop audio seek deferred. See
docs/chunking_v2.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_chunking_v2.py

@@ -0,0 +1,123 @@
+#!/usr/bin/env python3
+"""Plan-level validation for chunking v2 (docs/chunking_v2.md).
+
+Verifies the transformer now INCLUDES clips that span a chunk boundary and seeks
+into them (skip_override / src_offset), instead of the v1 behaviour that dropped
+them. This is a pure plan-level check — the ffmpeg concat-seam (frame alignment via
+-c copy) still needs a real render on the rig to confirm.
+
+Run:  ./venv/bin/python tests/test_chunking_v2.py
+"""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from gnommo.transformer import (
+    MarkerTiming,
+    _extract_audio_events,
+    _extract_video_events,
+    AUDIO_OFFSET_SECONDS,
+)
+from gnommo.models import AudioDefinition, VideoSource, CutoutDefinition, SlideDefinition
+
+_fails = []
+
+
+def check(name, cond, detail=""):
+    print(f"  {'PASS' if cond else 'FAIL'}  {name}" + (f"  — {detail}" if detail and not cond else ""))
+    if not cond:
+        _fails.append(name)
+
+
+# ── audio ────────────────────────────────────────────────────────────────────
+def test_audio():
+    print("audio events:")
+    audio = {
+        "music": AudioDefinition(file="music.mp3", loop=True, duration=90.0),
+        "sfx": AudioDefinition(file="sfx.wav", loop=False, duration=500.0),
+        "blip": AudioDefinition(file="blip.wav", loop=False, duration=50.0),
+    }
+    # music triggers at t=0, sfx at t=10, blip at t=10
+    markers = [
+        MarkerTiming(marker_id="Amusic", timestamp=0.0, context="", confidence=1.0),
+        MarkerTiming(marker_id="Asfx", timestamp=10.0, context="", confidence=1.0),
+        MarkerTiming(marker_id="Ablip", timestamp=10.0, context="", confidence=1.0),
+    ]
+
+    # Chunk window [300, 600): all three started earlier.
+    evs = {e.audio_id: e for e in _extract_audio_events(markers, audio, time_range=(300.0, 600.0))}
+
+    # Looping music: included, resumes at loop phase. astart = max(0, 0-1)=0; into=300;
+    # phase = 300 % 90 = 30.
+    check("looping music spanning boundary is INCLUDED (v1 dropped it)", "music" in evs)
+    if "music" in evs:
+        m = evs["music"]
+        check("music clamped to window start", abs(m.start_time - 300.0) < 1e-6, f"start={m.start_time}")
+        check("music seeks to loop phase 30.0", abs(m.src_offset - 30.0) < 1e-6, f"src_offset={m.src_offset}")
+
+    # One-shot still playing at the window: included, linear seek.
+    # astart = max(0,10-1)=9; aend=9+500=509 > 300 → spans. into=300-9=291.
+    check("one-shot still playing is INCLUDED", "sfx" in evs)
+    if "sfx" in evs:
+        s = evs["sfx"]
+        check("sfx linear seek 291.0", abs(s.src_offset - 291.0) < 1e-6, f"src_offset={s.src_offset}")
+
+    # One-shot that ended before the window: excluded (aend=9+50=59 < 300).
+    check("one-shot ended before window is EXCLUDED", "blip" not in evs)
+
+    # Full render (no range): everything from the start, no seek.
+    full = {e.audio_id: e for e in _extract_audio_events(markers, audio, time_range=None)}
+    check("full render includes music with no seek", "music" in full and full["music"].src_offset == 0.0)
+
+
+# ── video ────────────────────────────────────────────────────────────────────
+def test_video():
+    print("video events:")
+    cutouts = {"fullscreen": CutoutDefinition(x=0, y=0, height=1080, width=1920)}
+    videos = {
+        "bg": VideoSource(
+            source_file="bg.mp4", cutout="fullscreen", layer="below",
+            duration=90.0, skip=0.0, end_on="next_video",
+        )
+    }
+    slides = {f"S{i}": SlideDefinition(image=f"S{i}.png", type="slide") for i in range(1, 11)}
+    markers = [MarkerTiming(marker_id=f"S{i}", timestamp=(i - 1) * 60.0, context="", confidence=1.0)
+               for i in range(1, 11)]
+    # Background overlay starts at slide-7 time (360) and, as the only video with
+    # end_on next_video, runs to total_duration (600).
+    markers.append(MarkerTiming(marker_id="vfm:bg", timestamp=360.0, context="", confidence=1.0))
+
+    total = 600.0
+    # Chunk window that STARTS AFTER the video began: [420, 600) (slides 8-10).
+    evs, _ = _extract_video_events(markers, videos, cutouts, slides, total, time_range=(420.0, 600.0))
+    bg = next((e for e in evs if e.video_id == "bg"), None)
+
+    check("spanning background video is INCLUDED in the later chunk (v1 dropped it)", bg is not None)
+    if bg is not None:
+        check("bg clamped to window start", abs(bg.start_time - 420.0) < 1e-6, f"start={bg.start_time}")
+        # into = 420-360 = 60; playable = 90-0 = 90; 60 < 90 → linear seek 60.
+        check("bg linear seek 60.0 (first play-through)", abs((bg.skip_override or 0) - 60.0) < 1e-6,
+              f"skip_override={bg.skip_override}")
+
+    # Window starting deep enough that the 90s clip has looped once: [480, 600).
+    # into = 480-360 = 120; 120 >= 90 → phase = 120 % 90 = 30.
+    evs2, _ = _extract_video_events(markers, videos, cutouts, slides, total, time_range=(480.0, 600.0))
+    bg2 = next((e for e in evs2 if e.video_id == "bg"), None)
+    check("looped background resumes at phase 30.0", bg2 is not None and abs((bg2.skip_override or 0) - 30.0) < 1e-6,
+          f"skip_override={getattr(bg2, 'skip_override', None)}")
+
+    # Full render: no seek.
+    evs3, _ = _extract_video_events(markers, videos, cutouts, slides, total, time_range=None)
+    bg3 = next((e for e in evs3 if e.video_id == "bg"), None)
+    check("full render includes bg with no seek", bg3 is not None and bg3.skip_override is None)
+
+
+if __name__ == "__main__":
+    test_audio()
+    test_video()
+    print()
+    if _fails:
+        print(f"FAILED: {len(_fails)} check(s): {', '.join(_fails)}")
+        sys.exit(1)
+    print("All chunking-v2 plan-level checks passed.")