Price cycles in the mode decision: 37 misses become 1, for 0.26 dB

The decoder has been CPU-bound since FINDINGS 28 while the mode decision
minimised D + lam*R -- distortion against BYTES. decide() now minimises
D + lam*bytes + mu*cycles, and ratectl bisects mu per frame against the
833,333-cycle budget with the lam bisection nested inside it. On the worst
sustained window:

  sasi  27.22 -> 26.95 dB, 109.5 -> 109.4 KB/s, 37/120 misses -> 1
  scsi  29.90 -> 29.27 dB, 280.0 -> 278.6 KB/s, 51/120 misses -> 1

Bitrate does not move: the byte controller still binds, and mu changes WHICH
modes are bought. V4 is what it stops buying -- 25.2 -> 20.3% of blocks at sasi
and 15.0 -> 5.3% at scsi, where RAW takes it. That is 28.8's inversion in
practice: RAW is dearer in bytes and cheaper in cycles, so only the byte-rich
profile can buy its way out of V4.

Three things worth knowing beyond the headline:

  - The one frame that still misses, at both profiles, is FRAME 0 -- no previous
    reconstruction, so 100% changed by definition, which is also what a scene
    cut is. It comes out at the all-V1 floor of 110.6% and is emitted late on
    purpose. Freezing a cut to make a deadline is the worse failure.
  - 28.7's "11 frames are impossible" was too pessimistic. That floor held the
    SKIP set fixed and asked how cheaply the drawn blocks could be drawn; the
    real decision can also MOVE a block to SKIP, which above ~90% non-SKIP is
    the only lever left.
  - SKIP's price depends on its neighbours (13.25 cycles clustered, 45 mixed),
    which a per-block lagrangian cannot see. The way out is that the two uses
    need not share a cost function: a ranking constant inside decide(), the
    exact clustered rule for the frame-level bisection. vq_hybrid.cycles() is
    now the one definition of that rule and 11_cpu_budget.py imports it.

Gated: 09_ratectl_drift.py runs both controllers, both 0/120 drifting frames.
The cost-aware container decodes pixel-exact on the 68000 (120 frames). ON by
default in encode.py; --no-cpu-fit restores session 7. check.sh ALL GREEN.

Still a model, not a measurement, for THIS container: FINDINGS 31's cycle
figures come from vq_hybrid.cycles (within 1 point of the 68000 on four frames
of the session-7 container). Timing this one on the machine is step 1 of the
next session -- it was started and killed for time, and it is slow.

FINDINGS 31. tools/analysis/13_cpu_ratectl.py.

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

README.md

@@ -39,7 +39,9 @@ tools/analysis/  measurement scripts, numbered in the order they were written
                  11 scores a container against the MEASURED per-mode block
                  costs without needing MAME; 12 prices the literal-span mode of
                  FINDINGS 30 against those same mode maps, and prints whether a
-                 scene cut still fits at 12fps.
+                 scene cut still fits at 12fps; 13 measures what fitting the
+                 CPU budget costs in dB (FINDINGS 31) and caches H.build so the
+                 search loop is seconds, not minutes.
 tools/bench/     MAME Lua injection harness + 68000 benchmark sources.
                  `check.sh` re-runs both display regression tests (~40 s).
                  `blit.s`/`blit.lua` time the full-frame GVRAM blit on the
@@ -70,7 +72,14 @@ python3 tools/encoder/encode.py  /tmp/fr out.dlx --profile sasi --preview p.png
 Two quality profiles ship from one codec and one decoder — `sasi` (110 KB/s) and
 `scsi` (280 KB/s) are two points on the same rate-distortion curve. Both are
 **ceilings**: lam is bisected per frame under a leaky bucket, so the profile's
-`lam` is a quality floor rather than a setting (`--fixed-lam` opts out). The codec is
+`lam` is a quality floor rather than a setting (`--fixed-lam` opts out).
+
+There are **two** ceilings, on two different axes. The second is the 68000's
+decode budget: `mu` is bisected per frame against 833,333 cycles so the frame
+also *decodes* in time, which takes the worst sustained window from 37 frames
+over budget to 1 for 0.26 dB (FINDINGS 31). It is on by default; `--no-cpu-fit`
+restores session 7 behaviour. Unlike bytes, cycles have no bucket — there is no
+double buffer to decode ahead into, so it is a hard per-frame ceiling. The codec is
 a Cinepak-style hybrid: each 4x4 block is coded as SKIP, one 4x4 codeword, four
 2x2 codewords, or RAW literal pixels, chosen per block by rate-distortion.