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

docs/STATUS.md

@@ -156,6 +156,41 @@ functional models, not timing-accurate; a KB/s figure from MAME measures the
 emulator's scheduler. `docs/BENCHMARK.md` covers the three-tier approach
 (MAME validates the path, derivation bounds it, real hardware settles it).
 
+## Display path — WORKING, verified end to end (session 3)
+
+The first real frame is on screen: `docs/images/x68k_first_frame_compare.png`.
+Full write-up in **FINDINGS 22**. Harness: `tools/bench/show_frame.lua` +
+`tools/bench/prep_frame.py`.
+
+Three facts the player MUST honour, none of which were guessable:
+
+| what | where | value |
+|---|---|---|
+| **Un-hide the graphics layer** | CRTC R20 `$E80028` | clear bit 11 ("G-VRAM set to buffer"); IPL leaves `0x0B16` |
+| Colour setup (256c) | CRTC R20 bits 9-8 | `0x0100` -> `R20 = 0x0116` |
+| **Monitor contrast** | `$E8E001` bits 3-0 | IPL leaves **14**; write **15** or everything renders 7% dark |
+
+Bit 11 is the one that cost the most time: GVRAM writes land and read back
+correctly while the layer is invisible, so the video controller looks guilty and
+is not. Contrast `0` blanks the screen — free fade-to-black for transitions.
+
+Palette format is now **confirmed from MAME source**, not assumed:
+`GGGGGRRRRRBBBBBI` (G 15:11, R 10:6, B 5:1, shared LSB I), expanded as
+`pal6bit((field<<1)|I)`. With contrast at 15 the render is **pixel-exact**.
+
+New 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
+(39.4 dB). `scsi` is close to display-transparent on real hardware. See
+FINDINGS 22.4 before considering raising quality further.
+
+Snapshot recipe that works (`-video none` CANNOT snapshot):
+```
+SDL_VIDEODRIVER=dummy mame x68000 -bios ipl10 -video soft -window \
+  -sound none -nothrottle -plugins -autoboot_script <script>.lua \
+  -snapshot_directory ./snap -snapview native -seconds_to_run 6
+```
+`-snapview native` drops MAME's LED artwork and gives a clean 768x512 screen.
+
 ## Next steps, in priority order
 
 1. **Full-disc survey.** Only 4 clips of 1.2-1.7 s out of 224 streams have been
@@ -164,9 +199,12 @@ emulator's scheduler. `docs/BENCHMARK.md` covers the three-tier approach
    vs content first (FINDINGS 13) or the averages are diluted by static menus.
    **Vectorise `_paint` before this run** — it is a Python per-block loop.
 2. **68000 decoder skeleton.** Parse `DLX1`, expand codebooks to word-per-pixel,
-   blit SKIP/V1/V4/RAW. Measure real cycles with the existing MAME Lua harness —
-   the first time that harness gets used for its actual purpose. Validates the
-   38% full-frame blit estimate that the whole CPU budget rests on.
+   blit SKIP/V1/V4/RAW. Measure real cycles with the existing MAME Lua harness.
+   **Now unblocked** — the display path is verified (FINDINGS 22) and
+   `tools/bench/show_frame.lua` gives a known-good reference image to diff the
+   68000's output against. Validates the 38% full-frame blit estimate that the
+   whole CPU budget rests on. Still needs a real CRTC mode table for 256x256;
+   the harness deliberately borrows the IPL's timing and invents nothing.
 3. **Wire rate control into `encode.py`.** No longer a blocker (FINDINGS 21), but
    it is what gives a deterministic ceiling over content not yet measured, which
    was the original reason for choosing VQ. Insurance, not a fix. Pairs with (1).