A real 256x256 CRTC mode, derived not recalled; palette ceiling was 2 dB low

Session 3 left the harness on the IPL's 768x512 text timing because no CRTC
values had been derived and guessing them was the failure mode to avoid. This
derives them from MAME 0.277's divisor ladder instead, and the derivation is
self-checking: the 256-wide mode runs at div 6 against the 768 mode's div 2, so
htotal is exactly 1104/3 = 368 dots and every horizontal register divides by
three with no remainder. Only the blanking split rounds. Verified by snapshot:
native 256x512, active area pixel-exact, x=512 wrap gone.

Two things fell out that change numbers elsewhere:

- The palette's shared LSB I must be chosen per entry, not hardcoded to 1.
  Doing so lifts the display ceiling from 38.85 to 40.81 dB and is the only way
  to reach true black at all, since pal6bit(1) = 4. 102 of 256 entries want
  I = 0, so this is not a corner case. Supersedes FINDINGS 22.4; scsi has ~2 dB
  more headroom than that section claimed. The encoder does not do this yet.

- Letterboxing costs a palette entry: GVRAM cleared to zero shows entry 0, and
  a free mediancut palette puts a real image colour there. 255 colours plus a
  reserved black, via prep_frame.py --reserve-black.

MAME's graphics double-scan is phase-shifted one raster line (it halves the
absolute scanline and vbegin is odd), which produced a false failure before it
was understood; the regression test now asserts the shifted pairing explicitly.

Still Lua-side. No 68000 instruction has drawn a pixel; the 38% blit estimate
remains unvalidated. What this buys is a defined geometry for the decoder to
write into: 256 words per row, 1024-byte stride, rows 32..223.

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

tools/bench/verify_frame256.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""Regression test for the 256x256 CRTC mode (docs/FINDINGS 23).
+
+Checks tmp/snap256/x68000/0000.png against tmp/frame256.bin:
+  1. native snapshot is 256x512 -- 256 dots, and 512 active scanlines of a
+     568-line 31.5kHz raster carrying 256 double-scanned graphics rows
+  2. double-scan pairing is (1,2),(3,4),... -- MAME halves the ABSOLUTE
+     scanline (x68k_v.cpp get_gfx_pixel) and vbegin=41 is odd, so snapshot
+     row 0 is a lone half-line and even rows are gfx rows 0..255
+  3. the 192 active rows are PIXEL-EXACT against the palette pushed through
+     GGGGGRRRRRBBBBBI with I chosen per entry by minimum squared error
+  4. the letterbox bars are TRUE black -- needs both a reserved index-0 black
+     entry AND I=0 on it, since pal6bit(1) = 4, not 0
+"""
+import struct, sys
+import numpy as np
+from PIL import Image
+
+s = np.asarray(Image.open("tmp/snap256/x68000/0000.png").convert("RGB")).astype(int)
+d = open("tmp/frame256.bin", "rb").read()
+W, H = struct.unpack(">HH", d[4:8])
+pal = np.frombuffer(d[8:8+768], np.uint8).reshape(256, 3).astype(int)
+idx = np.frombuffer(d[8+768:8+768+W*H], np.uint8).reshape(H, W)
+
+p6 = lambda v: ((v << 2) | (v >> 4)) & 0xFF
+f = pal >> 3
+render = lambda I: p6((f << 1) | I[:, None])
+I = (((render(np.ones(256, int)) - pal) ** 2).sum(1)
+     < ((render(np.zeros(256, int)) - pal) ** 2).sum(1)).astype(int)
+exp = render(I)[idx]
+
+fail = []
+if s.shape[:2] != (512, 256):
+    fail.append(f"1. geometry: expected 512x256, got {s.shape[1]}x{s.shape[0]}")
+if not all(np.array_equal(s[i], s[i+1]) for i in range(1, s.shape[0]-1, 2)):
+    fail.append("2. double-scan pairing (1,2),(3,4),... broken")
+
+g = s[0::2]
+yoff = (g.shape[0] - H) // 2
+act = g[yoff:yoff+H]
+if not np.array_equal(act, exp):
+    diff = abs(act - exp)
+    fail.append(f"3. active area not pixel-exact: maxdiff {diff.max()}, "
+                f"{diff.any(2).sum()} px differ")
+
+bars = np.concatenate([g[:yoff], g[yoff+H:]])
+if bars.max() != 0:
+    fail.append(f"4. letterbox not true black: max channel {bars.max()}")
+
+for x in fail:
+    print("FAIL " + x)
+if fail:
+    sys.exit(1)
+
+mse = ((act - pal[idx]) ** 2).mean()
+print(f"OK  256x512 native, double-scan exact, active {W}x{H} pixel-exact, "
+      f"letterbox true black")
+print(f"    palette ceiling vs 24-bit palettised source: "
+      f"{10*np.log10(255**2/mse):.2f} dB  ({(I==0).sum()}/256 entries use I=0)")