Handoff: reconcile docs and tooling with the corrections made this session

Session 2 reversed several of its own conclusions. The docs are append-only, so
a reader could land on a superseded section and act on it. This pass makes the
repo internally consistent.

Defects found and fixed in STATUS.md:
- claimed "Hybrid VQ with k=1024: no" as the answer to the linework question,
  directly contradicting FINDINGS 14, which rejected k=1024. Both profiles are
  k=256.
- malformed profile table (six column separators, five columns).
- next-steps list had two items numbered 3 and listed the full-disc survey
  twice.
- the disk-benchmark section still read CRITICAL-PATH with "if SCSI sustains
  >=800 KB/s, ship pixel-exact". That was written while the bandwidth figure
  was misread as 4 MB/s. At 4 Mbps pixel-exact needs 92-97% of the pipe and is
  not available, and the ring-buffer result means the design no longer hangs on
  the benchmark at all. Rewritten with what it IS still worth doing: confirming
  the 4 Mbps provenance, and confirming DMA is used rather than PIO.

FINDINGS now carries supersession blockquotes on 5, 8, 11, 17 and 18 pointing
at the sections that correct them. 18 is the dangerous one -- its peak-vs-
sustained test is reversed by 21 -- so it is marked DO NOT ACT ON THIS SECTION
while noting the per-frame data itself remains valid.

profile_gen.py had the same problem in code: it defaulted to the superseded
peak sizing and returned lam=25 where the docs say lam=10. The buffered test is
now the default and peak sizing is behind --size-for-peak as a bound only. A
tool that contradicts the findings is worse than no tool.

Also preserves the five measurement scripts that produced this session's
numbers as tools/analysis/05-09, following the session 1 precedent, and adds an
"explicitly abandoned -- do not re-propose" list to STATUS covering entropy
coding, k=1024 codebooks and flat 4x4 VQ.

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

tools/encoder/profile_gen.py

@@ -11,12 +11,12 @@ bandwidth and it returns the lam that fits, with the headroom accounted for.
 Three things eat the pipe before video gets any:
 
   1. AUDIO         -- 7.8 KB/s of MSM6258 ADPCM, constant.
-  2. PEAK/MEAN     -- measured 1.4-1.9x (FINDINGS 18). The disk delivers a
-                      SUSTAINED rate; a frame that overruns is a DROPPED frame.
-                      Either size for the peak, or rate-control to the mean and
-                      carry a bucket. We do the latter, so we need bucket depth
-                      rather than peak headroom -- but until rate control is
-                      actually wired in (it is not), size for the peak.
+  2. BUFFERING     -- NOT peak/mean. FINDINGS 18 sized against the per-frame
+                      peak; 21 showed that is the wrong test. The disk keeps
+                      filling DURING a frame, so the condition is cumulative
+                      demand vs cumulative supply, which every measured scene
+                      passes with ZERO required prefill. Peak sizing is kept
+                      behind --size-for-peak only as a pessimistic bound.
   3. DMA CYCLE-STEAL -- the HD63450 steals ~8 clocks per 16-bit word from the
                       68000. At 488 KB/s that is 20% of the CPU, on top of the
                       blit. Bandwidth and CPU are NOT independent budgets.
@@ -49,16 +49,23 @@ CURVE = [
 ]
 CEILING = {"00020": 39.90, "00146": 35.25}
 PEAK_OVER_MEAN = 1.9              # measured worst case, FINDINGS 18
+# Per-frame fill at 12fps must cover the worst single frame, else the buffer
+# has to carry the difference. Measured worst frame is 42.10 KB (00146 lam=10).
+WORST_FRAME_KB = 42.10
 
 
 def dma_steal_pct(kbps):
     return (kbps * 1024 / 2) * DMA_CLOCKS_PER_WORD / CLK * 100
 
 
-def pick(bw_kbps, peak_factor=PEAK_OVER_MEAN, margin=0.85, rate_controlled=False):
-    """Largest-quality lam whose worst-case demand fits inside bw_kbps."""
+def pick(bw_kbps, peak_factor=PEAK_OVER_MEAN, margin=0.85, size_for_peak=False):
+    """Largest-quality lam whose sustained demand fits inside bw_kbps.
+
+    Default is the BUFFERED test (FINDINGS 21): compare mean demand against the
+    sustained fill. size_for_peak=True restores the pessimistic FINDINGS 18
+    sizing, which is retained only as a bound -- it is not the shipping rule."""
     usable = bw_kbps * margin - AUDIO_KBPS
-    factor = 1.0 if rate_controlled else peak_factor
+    factor = peak_factor if size_for_peak else 1.0
     allow_mean = usable / factor
     for lam, k20, d20, k146, d146 in CURVE:
         worst = max(k20, k146)
@@ -81,9 +88,9 @@ def main():
                     help="fraction of the pipe we allow ourselves (seeks, "
                          "container overhead, and the fact that the bandwidth "
                          "figure itself is folklore)")
-    ap.add_argument("--rate-controlled", action="store_true",
-                    help="assume the leaky bucket absorbs peaks (NOT YET TRUE "
-                         "-- ratectl.py is written but not wired into encode.py)")
+    ap.add_argument("--size-for-peak", action="store_true",
+                    help="pessimistic FINDINGS 18 sizing (mean * 1.9). Superseded "
+                         "by 21 -- kept only as a bound, not the shipping rule.")
     a = ap.parse_args()
 
     bw = a.bw_kbps if a.bw_kbps else a.bw_mbps * 1_000_000 / 8 / 1024
@@ -91,26 +98,33 @@ def main():
     print(f"bandwidth {src} = {bw:.0f} KB/s sustained")
     print(f"  usable at {a.margin:.0%} margin : {bw*a.margin:.0f} KB/s")
     print(f"  less audio ({AUDIO_KBPS})     : {bw*a.margin-AUDIO_KBPS:.0f} KB/s for video")
-    if not a.rate_controlled:
-        print(f"  less peak/mean {PEAK_OVER_MEAN}x    : "
-              f"{(bw*a.margin-AUDIO_KBPS)/PEAK_OVER_MEAN:.0f} KB/s mean allowance")
+    fill_per_frame = bw / FPS
+    if a.size_for_peak:
+        print(f"  less peak/mean {PEAK_OVER_MEAN}x   : "
+              f"{(bw*a.margin-AUDIO_KBPS)/PEAK_OVER_MEAN:.0f} KB/s mean allowance"
+              f"   [pessimistic, FINDINGS 18 -- superseded]")
     else:
-        print("  peaks absorbed by rate control (bucket depth must be validated)")
+        print(f"  fill per frame time   : {fill_per_frame:.2f} KB "
+              f"(worst measured frame {WORST_FRAME_KB:.2f} KB"
+              f"{' -- COVERED' if fill_per_frame >= WORST_FRAME_KB else ' -- needs buffer'})")
 
-    r = pick(bw, margin=a.margin, rate_controlled=a.rate_controlled)
+    r = pick(bw, margin=a.margin, size_for_peak=a.size_for_peak)
     if r is None:
         print("\n  NO PROFILE FITS -- even lam=800 overruns. Lower the framerate,")
         print("  the resolution, or get more bandwidth.")
         return
-    steal = dma_steal_pct(r["peak_kbps"])
-    print(f"\n  -> {a.name}: lam={r['lam']}, {r['mean_kbps']:.0f} KB/s mean, "
-          f"{r['peak_kbps']:.0f} KB/s peak")
+    steal = dma_steal_pct(r["mean_kbps"])
+    print(f"\n  -> {a.name}: lam={r['lam']}, {r['mean_kbps']:.0f} KB/s mean")
     print(f"     quality  00020 {r['psnr20']:.2f} dB (-{r['loss20']:.2f} from ceiling)")
     print(f"              00146 {r['psnr146']:.2f} dB (-{r['loss146']:.2f} from ceiling)")
     print(f"     CPU      blit {BLIT_FULL_FRAME_PCT:.0f}% + DMA steal {steal:.1f}% "
           f"= {BLIT_FULL_FRAME_PCT+steal:.0f}% of the frame budget")
     if BLIT_FULL_FRAME_PCT + steal > 85:
         print("     WARNING: CPU is now the binding constraint, not the bus.")
+    if not a.size_for_peak and fill_per_frame < WORST_FRAME_KB:
+        short = WORST_FRAME_KB - fill_per_frame
+        print(f"     note: worst frame exceeds one frame-time of fill by "
+              f"{short:.2f} KB -- buffer must carry it (2MB RAM, non-issue).")
 
 
 if __name__ == "__main__":