Put the ring on the 68000, and find the disc stops whenever the player is not asking

ROADMAP P5. The loader moved in session 21 and the frame clock in 22; the ring
producer was the last policy living outside the machine. src/player/ring.i does
`aligned` placement, the descriptor ring, a prefill, 51.2's slack rule and a
seek, and the host keeps only the transport.

It needed a container change. `aligned` asks whether the next record fits
before the end of the ring -- a length asked BEFORE the record is fetched -- and
every reader in this tree answered that by walking the frame stream, which is
exactly what a player streaming off a disc cannot do. DLX4 carries nframes u16
record lengths in the scene header. Frame payloads are byte-identical to the
DLX3 encode, so no fitted constant moves; the scene header goes 5,920 to 6,164 B.

The producer reproduces the host's tiling exactly: 18 wraps, 14.7 KB mean hole,
pixel-exact, a third independent implementation of the same policy.

What it exposed is bigger than the item. A channel only moves bytes while it has
a request and only the CPU can issue one, so the disc stands still between
records by an amount the PLAYER sets, not the medium -- and no host-filled run
could see it. At 488 KB/s in a 256 KB ring a one-deep request queue gives away
6.8% of the pipe and underruns 59 of 120 frames; two-deep gives away 3.4% and
underruns none. The container's whole surplus over the wire is 8.7%, so the
player's own loop was spending most of the slack a branch point saves up.
Prefill is the weaker lever: six records of it still leaves 24 underruns.

Three silent bugs are recorded in FINDINGS 55.7 -- all produced wrong pixels or
a desync rather than a fault -- plus a rig one: MAME renders a screen line by
line, so snapshotting the frame the decoder finished in captures a tear that
reads exactly like a decoder bug.

check.sh gains the machine-owned ring and a seek with the decode after it.
decode.bin is unchanged at 1,296 B and a host-filled run executes none of the
new code, so every FINDINGS 49/51 figure stands. ALL GREEN before and after.

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

tools/analysis/16_span_roundtrip.py

@@ -72,8 +72,15 @@ for span_mode in ("need", "all"):
           f"({100*px/(len(recs)*m['H']*m['W']):.1f}% of all pixels)")
 
     d = DLX(path)
-    if d.version != 3:
-        print(f"FAIL: container is DLX{d.version}, not DLX3"); bad += 1; continue
+    if not d.has_spans:
+        print(f"FAIL: container is DLX{d.version}, which has no span section")
+        bad += 1; continue
+    # DLX4 adds the record index and DLX() cross-checks it against its own walk
+    # 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 "
+              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
     # block payload starts, and blocks() already raises if the payload does not
@@ -104,5 +111,5 @@ print()
 if bad:
     print(f"FAILED: {bad} check(s)")
     sys.exit(1)
-print("OK  the DLX3 span container round-trips: the reference decoder rebuilds "
+print("OK  the span container round-trips: the reference decoder rebuilds "
       "the\n    encoder's reconstruction exactly, from the emitted bytes.")