Get a real Dragon's Lair frame onto the emulated X68000

First pixels on an actual X68000 screen. Everything up to now was Python-side
or a headless -video none run, which cannot snapshot at all.

The blocker was not the video controller. The IPL leaves CRTC R20 = 0x0B16,
and bit 11 is "G-VRAM set to buffer", which makes MAME's draw_gfx() return
early. GVRAM writes still land and read back correctly while the layer is
invisible, so six attempts at $E82400/$E82500/$E82600 all rendered black with
every register holding the value I intended.

Two more facts, both confirmed against MAME 0.277 source rather than assumed:

- $E8E001 monitor contrast is left at 14 by the IPL, scaling all output to
  93.3%. The player must set it to 15. Contrast 0 blanks the screen, which is
  a free fade-to-black for scene transitions.
- The palette word is GGGGGRRRRRBBBBBI with a shared LSB, expanded as
  pal6bit((field<<1)|I). With contrast at 15 the render is pixel-exact, not
  merely close, which also confirms the 1024-byte GVRAM line stride.

That exactness gives a new quality ceiling: the 15-bit+I palette alone costs
38.88 dB against the 24-bit palettised source, the same order as the scsi
profile's own codec error. scsi is close to display-transparent on hardware,
which bounds how much further it is worth raising.

Unblocks next step 2, the 68000 decoder skeleton, which now has a known-good
reference image to diff against.

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

.gitignore

@@ -12,3 +12,4 @@ __pycache__/
 *.pyc
 *.dlx
 a.out
+tmp/