Build v7 into the player, and find the cost model 18% wrong on the block it made commonest

src/player/decode.s now paints v7 literal spans, pixel-exact under MAME and
px68k's C68K core over a container where every frame carries 128-216 spans
covering up to 38% of the picture. The span pass is blit.s v7 verbatim: the
66.0/9.143/9.978 fit was measured on that instruction sequence.

The container is DLX3 -- a span section between the mode header and the block
payload, since that is the only place the 68000 can reach without first parsing
something of variable length. 16_span_roundtrip.py gates it in check.sh, and
asserts it emitted enough spans to have tested anything.

Two synthetic all-SPAN anchors price v7 inside decode.s at 151.2 and 225.6
clocks per 4x4 block, against FINDINGS 40's table of 151 and 226 -- 0.2% on
both emulators. The measured mode costs what it was said to cost.

Two things that were not on the list:

TWO BYTE BUDGETS. FINDINGS 40's 18/120 was scored against the 488 KB/s PIPE,
not the 280 KB/s profile, and at the profile rate the lam search has already
spent the allowance -- spans fired on 5 frames of 120 and looked like a
regression. The profile is a chosen quality rate point; the pipe is hardware.
--kbps and --span-kbps are now separate and spans run before mu, because a span
pays in bytes and mu pays in picture. Delivered: 86/120 over budget without
spans, 77/120 at the profile budget, 34/120 on the pipe for +0.36 dB.

C_SKIP_MIXED WAS NEVER MEASURED, and it was 18% low -- 45.0, now 55.0. It is
the one constant in the table that came from a derivation, because the
synthetic frame that would measure it cannot exist: a byte needs a coded block
for its SKIP to be mixed. Four bracketing anchors measure it on both emulators
with the header byte rotated through all four positions, and the partner mode
solves back to its own anchored value to 0.2%. With it corrected the model
predicts a real spanned decode to -0.06% mean / 0.09% worst, against -2.99% /
4.30%. It matters because a span marks its run SKIP, so mixed SKIPs dominate
exactly the frames spans are judged on.

Also: the rig had been writing its synthetic timing frames 26 KB past the top
of a 2 MB machine, and got away with it because the modes it overran are
data-independent. A span's jump displacements come out of the stream, so it is
not. And frames-over-budget is no longer a safe headline -- the controller aims
at the deadline, so 55 of 120 frames sit within 5% of it and a 1% cost shift
moves 22 frames.

FINDINGS 41. check.sh ALL GREEN, now gating on a span-heavy DLX3 container.

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

tools/encoder/vq_hybrid.py

@@ -61,7 +61,30 @@ RAW_BYTES = 16.0                  # literal palette bytes, never indices
 #     mode decision uses the ranking constant.
 C_V1, C_V4, C_RAW = 299.9, 448.2, 400.4
 C_SKIP_CLUSTERED = 53.0 / 4       # all-SKIP header byte: one tst.b for four
-C_SKIP_MIXED = 45.0               # a SKIP block inside a mixed byte
+# C_SKIP_MIXED WAS THE ONE CONSTANT HERE THAT HAD NEVER BEEN MEASURED. It was
+# 45.0, hand-derived, from session 7 until session 12 measured it -- and it was
+# 18% low. Every other figure in this table comes from a synthetic frame of a
+# single mode, and there was no such frame for a SKIP in a MIXED byte, because
+# a frame of nothing but mixed SKIPs cannot exist: the byte has to hold a coded
+# block for the SKIP to be mixed at all.
+#
+# tools/bench/prep_dlx.py now emits four that bracket it -- (3 SKIP + 1 V1),
+# (1 SKIP + 3 V1), (3 SKIP + 1 RAW), (1 SKIP + 3 RAW), each with the header byte
+# ROTATED through all four positions so no mode is pinned to the free `lsr`
+# slot -- and each pair solves for the SKIP cost and its partner's together:
+#
+#            MAME     C68K        (the partner solves back to its own anchored
+#   V1 pair  55.03    56.50        value to 0.2%, which is what says the pair
+#   RAW pair 55.83    56.50        is measuring the SKIP and not absorbing it)
+#
+# 55.0 is taken because every other constant here is MAME's; C68K reads V4 and
+# RAW 3.2-3.5% higher on pure frames too, which is FINDINGS 37's known table
+# spread and not a property of mixed bytes.
+#
+# It matters more than 10 clocks a block sounds, because a v7 SPAN marks its
+# run SKIP: a spanned container is made largely of mixed SKIPs, so this is the
+# dominant population in exactly the frames spans are judged on. FINDINGS 41.5.
+C_SKIP_MIXED = 55.0               # a SKIP block inside a mixed byte, MEASURED
 C_SKIP_RANK = C_SKIP_CLUSTERED    # ranking only -- see above
 MODE_CYCLES = np.array([C_SKIP_RANK, C_V1, C_V4, C_RAW], dtype=np.float64)