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/bench/dma_run.sh

@@ -76,6 +76,38 @@ grep -aq "COC .*CER=\$00 MTC=0 .*(+2048) \[held\]" tmp/dma_run.log || \
   fail "the held channel did not report a clean completion of every byte."
 grep -aq "COC .*CER=\$00 MTC=0 .*(+2048) \[steal\]" tmp/dma_run.log || \
   fail "the stealing channel did not report a clean completion of every byte."
+# ---- the GVRAM run and its control (47.6.2). A channel that writes GVRAM in
+# buffer mode is the decoder-free packed player's entire per-frame path, and a
+# run with no control is 58.3's vacuous "UNDERRUNS: 0/120" again -- the IPL
+# leaves R20 = $0B16, bit 11 ALREADY SET, so the first cut of this test could
+# not have failed.
+grep -aq "BYTES OK: 2048 B from LBA 1000 .*\[gvram\]" tmp/dma_run.log || \
+  fail "the channel did not fill GVRAM in buffer mode -- a device->GVRAM
+      transfer is the whole of the decoder-free packed player's frame."
+grep -aq "R20 during the run = \$0916 (bit 11 SET)" tmp/dma_run.log || \
+  fail "the GVRAM run did not run in buffer mode with a KNOWN R20."
+grep -aq "R20 during the run = \$0116 (bit 11 CLEAR)" tmp/dma_run.log || \
+  fail "the negative control did not run with bit 11 clear."
+if grep -aq "BYTES OK: 2048 B from LBA 1000 .*\[masked\]" tmp/dma_run.log
+then
+  fail "the MASKED control delivered every byte, so the run above is not a
+      measurement of R20 bit 11 -- it is a measurement of nothing."
+fi
+grep -aq "EXACTLY THE MECHANISM" tmp/dma_run.log || \
+  fail "the masked control lost bytes at ODD offsets too, or lost none at all.
+      The claim is not a COUNT -- stale GVRAM matches the disc by coincidence
+      wherever the record is pad -- it is a PLACE: gvram_w's 256-colour arm
+      drops what the channel wrote to EVEN addresses and stores what it wrote
+      to odd ones. Damage anywhere else is a different mechanism."
+
+grep -aq "BYTES OK: 2048 B from LBA 1000 .*\[chain\]" tmp/dma_run.log || \
+  fail "the array-chained run did not put the bytes at the row bases its array
+      named. A picture row is 256 B of a 1024 B line stride, so a frame is 192
+      destinations; if the channel cannot walk them the CPU has to restart it
+      per row and the decoder-free path costs a per-row front end."
+grep -aq "THE CHANNEL WALKED THE ARRAY ITSELF" tmp/dma_run.log || \
+  fail "the chained run did not report walking its own array."
+
 grep -aq "WINDOWED DMA READ REFUSED" tmp/dma_run.log || \
   fail "a WINDOWED read through the channel was not refused. 117 of 120 records
       start part way into a sector (58.3), and a channel cannot drop the bytes