Find the sustained action sequence: it breaks both profiles

The open risk since session 2 was "a sustained action sequence could still
break the bitrate", with every clip measured so far being 1.2-1.7 s. Closed by
measurement rather than by sampling clips by hand.

07_motion_survey.py scans a whole stream at 96x72 for the hottest sliding
window of inter-frame difference. On 00223 the spread between the quietest and
hottest sustained 10 s windows is 10.6x, which is the argument for not eyeballing
it. Hottest is t=539.4s, the Singe endgame.

There, with the fixed lam the CLI uses, sasi overshoots 110 -> 129.6 KB/s (+18%)
and scsi 280 -> 373.8 KB/s (+34%). Rate control moves from "insurance, not a
fix" to required, and is promoted above the full-disc survey. The bus is not
broken -- 381.6 KB/s still fits the 488 KB/s figure -- so FINDINGS 21 survives,
at 78% of the pipe instead of a comfortable margin.

Three further corrections fall out:

- The two largest streams on the disc are bonus material. 00216 is the feature
  with a burned-in commentary PiP; 00215 is the commentary. 00223 is the clean
  9.4 min. A size-ranked survey would have encoded live action.
- On hard content the 256-colour scene palette (31.33 dB) binds well before the
  X68000 display (40.81 dB); scsi is already within 0.51 dB of it.
- FINDINGS 24.5's architecture question resolves to "both paths, chosen per
  frame": 30-53% of frames sit above the 70% crossover. Picking per frame costs
  a median 37.0% of the frame budget and caps at 53.6%. Reporting for this is
  wired into encode.py, which previously only printed a mean over all frames --
  the one statistic that cannot answer a per-frame question.

extract.py takes optional start/dur; 08_mode_map.py renders source | decoded |
block-mode map to .webm.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6

tools/encoder/extract.py

@@ -35,7 +35,13 @@ def extract(stream, outdir, fps=12, mode="crop", start=None, dur=None):
     return n
 
 if __name__ == "__main__":
+    # extract.py <stream> <outdir> [fps] [mode] [start_s] [dur_s]
+    # start/dur matter for the long streams: 00223 is 9.4 min and the parts
+    # that stress the codec are a few seconds each (tools/analysis/07 finds
+    # them). Without them a survey silently averages action into idle scenes.
     stream, outdir = sys.argv[1], sys.argv[2]
     fps = int(sys.argv[3]) if len(sys.argv) > 3 else 12
     mode = sys.argv[4] if len(sys.argv) > 4 else "crop"
-    extract(stream, outdir, fps, mode)
+    start = float(sys.argv[5]) if len(sys.argv) > 5 else None
+    dur = float(sys.argv[6]) if len(sys.argv) > 6 else None
+    extract(stream, outdir, fps, mode, start, dur)