Align the container to the disc, and find the decoder-free packed player fits

Two sessions, unrecorded until now, committed together because their edits
share files and cannot be split cleanly after the fact.

Session 28 (FINDINGS 60): the container is DLX5 -- every record sector-aligned,
120/120 starting on a boundary where 3/120 did, +0.48% on the wire and zero
clocks -- and the ring's release rounds to RECALN so no pad is stranded.  Two
encoder levers measured and refused: `--spans all` buys +0.19 dB for +67% of
the wire, and joint span/lam selection emits byte-identical containers because
`lam` never leaves its floor on any of 120 frames.

Session 29 (FINDINGS 61): the packed full-frame blit is 27.3% of a 12 fps
frame, a channel fills GVRAM in buffer mode off the disc with the CPU halted,
and it walks the 1,024 B line stride itself through array chaining.  At the
9 clk/B dual-address floor the codec is 110.4% of a frame and a decoder-free
packed literal player is 55.2%, at +4.89 dB -- 2.75 dB past a ceiling the
codec's scene-wide palette cannot cross.  Encoder work is parked; the codec is
kept and not built on.

check.sh is ALL GREEN before and after, plus one new stage that gates the ORDER
of the measured paint costs rather than their values.

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

tools/analysis/16_span_roundtrip.py

@@ -43,12 +43,26 @@ ap.add_argument("--cache", default=None)
 a = ap.parse_args()
 
 cache = a.cache or f"tmp/model_{os.path.basename(a.frames_dir.rstrip('/'))}.pkl"
+# The cache is keyed on the frames directory ALONE, which was fine while
+# H.build had no options and became a trap the moment it did: session 28's
+# reserved black entry (23.4) changes the palette, the codebooks and every
+# index in the model, and a pickle from before it would have let this gate
+# round-trip a container the shipping encoder no longer emits -- green, and
+# testing the wrong artefact.  So the build parameters are stored WITH the
+# model and a mismatch rebuilds.
+SIG = dict(k1=256, k4=256, iters=16, reserve_black=True)
+m = None
 if os.path.exists(cache):
     m = pickle.load(open(cache, "rb"))
-    print(f"model from {cache}")
-else:
+    if m.get("sig") != SIG:
+        print(f"{cache}: built with {m.get('sig')}, wanted {SIG} -- rebuilding")
+        m = None
+    else:
+        print(f"model from {cache}")
+if m is None:
     t = time.time()
-    m = H.build(a.frames_dir, k1=256, k4=256, iters=16)
+    m = H.build(a.frames_dir, **SIG)
+    m["sig"] = SIG
     pickle.dump(m, open(cache, "wb"))
     print(f"built model in {time.time()-t:.0f} s -> {cache}")
 
@@ -79,7 +93,7 @@ for span_mode in ("need", "all"):
     # of the frame stream, so simply constructing it above has already gated
     # that. Said out loud here because it is easy to read this as version drift.
     if d.has_index:
-        print(f"  DLX4: record index agrees with the frame stream on all "
+        print(f"  DLX{d.version}: record index agrees with the frame stream on all "
               f"{d.nframes} records ({2*d.nframes:,} B of scene header)")
 
     # The decoder's own walk of the span section must land exactly where the