Move the loader onto the 68000, and find 5,920 bytes nobody counted

src/player/load.i expands both codebooks to word-per-pixel form and packs the
palette to GGGGGRRRRRBBBBBI out of the RAW container header, byte-exact against
tools/bench/dlxload.py on both CPU cores.  The palette half is gated on words
read back out of the palette registers at $E82000, so "the words reached the
hardware" is part of what passes.  ROADMAP P1 is done; P2's encoder half (a
reserved black entry, 23.4) is not, and is a re-encode rather than an edit.

A scene change costs 18.96 ms of 68000 time, 22.8% of one 12 fps frame; boot
costs 24.70 ms.  The scratch tables describe the CRTC, not the scene, so
pal_tables is a separate entry point built once at boot -- 5.29 ms off every
scene change.

The one that moves something: the scene header is 5,920 B that no rate table in
this tree included, because it belongs to no frame record.  In FINDINGS 51.3's
currency it is divided by the surplus pipe - wire, so it is hypersensitive:
138 ms of extra refill climb at 488 KB/s and 1.099 s at 451.4 KB/s, for the
same bytes.  tools/analysis/22_scene_load.py prices it across explicit rates.

Recorded as open: the two CPU cores agree to <3% on every stage but the table
build, where they differ by 16.4%.  px68k's C68K charges a flat 50 clocks for
MULU/MULS (c68kmacro.h:1869) where the 68000 charges 38+2n, which explains
4,608 of the 8,703 clock gap.  4,095 clocks are unexplained.  Nothing else in
src/player/ multiplies, so no figure in FINDINGS 24-52 is affected.

decode.s and stream.s are untouched; decode.bin is still 1,296 B at the same
MD5.  check.sh gains a stage that gates byte-exactness on both cores and
deliberately does not gate the cycle counts -- MAME's clock is 1/55.46 s and a
wall timing would make the green light host-sensitive.

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

tools/analysis/22_scene_load.py

@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+"""What a scene change costs, now that the loader runs on the 68000 (FINDINGS 53).
+
+    python3 tools/analysis/22_scene_load.py [container ...] --kbps R [R ...]
+
+ROADMAP P1 asked for the codebook expansion to be priced "against the refill
+climb, not treated as free setup", and that is the whole job of this file.  A
+scene change is the one moment where every cost in this project lands at once:
+the ring is empty because of the seek, the header has to arrive before a single
+frame can be drawn, and the 68000 cannot decode anything until it has expanded
+the codebooks out of that header.
+
+THREE COSTS, IN THREE DIFFERENT UNITS, and they are not interchangeable:
+
+  * BYTES.  The container's header region -- palette, CB1, CB4 -- must be
+    delivered before frame 0 can be decoded.  It is not part of any frame
+    record, so no rate table in this tree has ever counted it.
+  * CLOCKS.  What src/player/load.i costs to turn that header into what the
+    block loop reads, MEASURED on the emulated 68000 by tools/bench/load.lua
+    and parsed out of its log rather than copied in here as a constant.
+  * ACCUMULATED SLACK.  The bytes above are bytes the pipe did not spend
+    filling the ring, so they cost play-time at the surplus rate (pipe - wire),
+    which is the currency FINDINGS 51.3 established a branch point spends.
+    This is the one that compounds: it is charged on top of the seek itself.
+
+`--kbps` is REQUIRED and takes no default, for the reason FINDINGS 50 gives.
+Rates are decimal-KB per the rest of the tree's tooling; sizes are KiB.
+"""
+import sys, os, re, argparse
+sys.path.insert(0, "tools/encoder")
+import numpy as np
+from dlx import DLX
+import ratectl as RC
+
+FPS = 12
+CPUHZ = 10_000_000
+
+
+def rig_cycles(path):
+    """The measured per-stage cost, out of tools/bench/load.lua's own log.
+
+    Parsed rather than pasted: a constant copied in here would go stale the
+    first time load.i changed, and it would go stale SILENTLY -- the arithmetic
+    below would keep working and keep being wrong.
+    """
+    if not os.path.exists(path):
+        sys.exit(f"no rig log at {path} -- run tools/bench/load_run.sh first")
+    out = {}
+    for line in open(path, "rb").read().decode("utf-8", "replace").splitlines():
+        m = re.search(r"^\[LOD\]\s+(\S.*?)\s{2,}(\d+) cyc", line)
+        if m:
+            out[m.group(1).strip()] = int(m.group(2))
+    need = ("SCENE CHANGE: codebooks + palette", "scratch tables only (boot, once)")
+    for k in need:
+        if k not in out:
+            sys.exit(f"{path} has no '{k}' line -- is it a load.lua summary?")
+    return out
+
+
+ap = argparse.ArgumentParser()
+ap.add_argument("containers", nargs="*", default=["tmp/rc_fr_singe_scsi_span.dlx"])
+ap.add_argument("--kbps", type=float, nargs="+", required=True,
+                help="delivered pipe rates, KB/s. REQUIRED, no default (FINDINGS 50)")
+ap.add_argument("--log", default="tmp/load_check.log",
+                help="tools/bench/load.lua's log, for the measured cycle counts")
+a = ap.parse_args()
+
+cyc = rig_cycles(a.log)
+scene_cyc = cyc["SCENE CHANGE: codebooks + palette"]
+boot_cyc = cyc["scratch tables only (boot, once)"]
+frame_cyc = CPUHZ / FPS
+
+print(f"measured on the emulated 68000 ({a.log}):")
+print(f"  per scene change   {scene_cyc:>8,} clocks = {1000*scene_cyc/CPUHZ:6.2f} ms "
+      f"= {100*scene_cyc/frame_cyc:.1f}% of one {FPS}fps frame")
+print(f"  once at boot       {boot_cyc:>8,} clocks = {1000*boot_cyc/CPUHZ:6.2f} ms "
+      f"  (the three scratch tables: scene-independent)")
+
+for path in a.containers:
+    d = DLX(path)
+    hdr = int.from_bytes(d.raw[28:32], "big")
+    rec = np.array([4 + n + (-(4 + n) % 4) for _, n in d.frames], np.int64)
+    wire = rec.mean() * FPS / 1024 + RC.AUDIO_KBPS
+    print(f"\n=== {path}: header region {hdr:,} B "
+          f"(pal 768 + cb1 {d.k1*16:,} + cb4 {d.k4*4:,} + 32), wire {wire:.1f} KB/s")
+    print(f"{'pipe':>6} {'header ms':>10} {'+load ms':>9} {'total':>7} "
+          f"{'frames':>7} {'surplus':>9} {'slack s':>9}")
+    for kbps in a.kbps:
+        hdr_ms = 1000 * hdr / (kbps * 1024)
+        load_ms = 1000 * scene_cyc / CPUHZ
+        total = hdr_ms + load_ms
+        surplus = kbps - wire
+        # What the header costs in the currency of 51.3: play-time at the
+        # surplus rate.  A negative surplus means the container does not fit the
+        # pipe at all and no amount of play buys the bytes back.
+        slack = f"{hdr/(surplus*1024):8.3f}" if surplus > 0 else "  NEVER"
+        print(f"{kbps:>6.0f} {hdr_ms:>10.2f} {load_ms:>9.2f} {total:>7.2f} "
+              f"{total/(1000/FPS):>7.2f} {surplus:>9.1f} {slack:>9}")
+
+print("""
+Reading it.  The 'frames' column is the scene change's FIXED cost in 12fps
+frame slots, before the ring has been given a single frame of lookahead -- so it
+is a floor under the black gap at a branch point, not the gap itself.  The
+'slack s' column is the one that compounds with FINDINGS 51.3: the header's
+bytes are bytes that did not go into the ring, so they lengthen the climb back
+to the seek-slack ceiling by that much play-time, every time.""")