Drop SASI on capacity, then find the budget never had the disk in it

USER DECISION: drop the `sasi` profile. Not on bandwidth -- on capacity. A SASI
volume is 40 MB, and the 22.8 min of unique scene footage on the source Blu-ray
(streams 00000-00201, measured, not recalled) is 146 MiB at the LOWEST rate this
codec makes -- more than the machine's whole 4-unit SASI space. `scsi` is the
only profile now. FINDINGS 32.

Then the user asked whether we were drawing the wrong conclusions about PIO vs
DMA, and we were, more broadly than the question implied. Every CPU figure in
FINDINGS 24-34 is scored against the full 833,333 cycles/frame with nothing
subtracted for moving the bitstream off disk. Debiting the HD63450 cycle-steal
at the long-standing 8 clk/word ESTIMATE, "1 frame of 120 misses" becomes 84 of
120, median 112.4%. PIO at the span rate is 99.8% of the machine. Spans buy
cycles by spending bandwidth and the bandwidth returns as steal, so 31.6's "fits
completely" becomes a worst frame of 114.3%. 10 fps absorbs it: median 93.7%,
1/120. FINDINGS 35. `11_cpu_budget.py` takes --io dma|pio|none, defaults to dma,
and warns if asked for none.

Also landed:
- item 1 done: the cost model checked against the 68000 on a cost-aware
  container, -3.07% to +0.01%, whole-window mean -1.22%. FINDINGS 34.
- item 4 done: the container carries its own 4-byte record alignment (DLX2).
  94/120 record starts were on odd addresses -- an address error, not a slow
  read -- now 0/120 for 16 B/s. Re-encoding reproduces 31.1 exactly. FINDINGS 33.
- a `scsi` window does not fit the 2 MB machine the rig emulates (2.84 MB of
  stream past a 0x200000 ceiling). The gate now verifies 80 of 120 frames and
  SAYS so, and fails loudly when the pass does not complete, instead of
  reporting a phantom 49,005-pixel diff. FINDINGS 36.

Three near-misses this session had one shape: an unobservable run nearly
produced a false finding. stdbuf -oL on any MAME job that prints progress -- a
file is block-buffered too, and a run that is merely finishing looks exactly
like one that is wedged.

check.sh ALL GREEN.

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

tools/encoder/dlx.py

@@ -23,8 +23,15 @@ class DLX:
     def __init__(self, path):
         self.raw = open(path, "rb").read()
         b = self.raw
-        if b[:4] != b"DLX1":
-            raise ValueError(f"{path}: not a DLX1 container")
+        # DLX2 pads every frame record up to a 4-byte boundary; DLX1 lays them
+        # end to end.  On a 68000 that is not a slow read but an ADDRESS ERROR
+        # (FINDINGS 28.3), so the padding is part of the format, not a loader
+        # convenience -- but DLX1 containers stay readable, because every
+        # measurement in FINDINGS 28-31 was taken on one.
+        if b[:4] not in (b"DLX1", b"DLX2"):
+            raise ValueError(f"{path}: not a DLX container")
+        self.version = int(b[3:4])
+        self.aligned = self.version >= 2
         (self.W, self.H, self.fps, self.nframes,
          self.k1, self.k4) = struct.unpack(">HHHHHH", b[4:16])
         off_pal, off_cb1, off_cb4, off_frm = struct.unpack(">IIII", b[16:32])
@@ -41,14 +48,22 @@ class DLX:
         self.mode_bytes = (self.nb * 2 + 7) // 8
 
         # frame directory: (offset of the mode header, payload length)
+        if self.aligned and off_frm % 4:
+            raise ValueError(f"{path}: DLX2 frame stream starts at {off_frm}, "
+                             f"which is not 4-byte aligned")
         self.frames = []
         p = off_frm
         for _ in range(self.nframes):
             (n,) = struct.unpack(">I", b[p:p + 4])
             self.frames.append((p + 4, n))
             p += 4 + n
-        if p != len(b):
-            raise ValueError(f"{path}: {len(b) - p} trailing bytes after "
+            if self.aligned:
+                p += -p % 4          # skip the pad to the next record
+        # The writer does not pad after the LAST record -- nothing follows it --
+        # so `p` may have advanced past the end by up to 3 bytes there.
+        slack = len(b) - p
+        if not (slack == 0 or (self.aligned and -3 <= slack < 0)):
+            raise ValueError(f"{path}: {slack} trailing bytes after "
                              f"{self.nframes} frames")
 
     def modes(self, f):