[belikh]

IRIX 7's kernel clears the overlay, popup and CID planes with three full-screen 1280x1024 block fills at the top of gfx::init, before VC2/XMAP/RAMDAC programming. A bug in that shape traps the guest with no graphics output, so it is worth pinning exactly.

The test runs the kernel's register sequence (DRAWMODE1 planes 4/5/6 with 24-bit RGB, WRMASK 0x00FF_FFFF, zero colours, XYENDI 1279,1023, XYSTARTI 0,0, DRAWMODE0 = 0x0306) on both engines and requires:

• the compiled shader matches the interpreter pixel-for-pixel (the full 8 MB aux buffer is compared);
• the fill clears the 24 plane bits of every screen pixel and leaves the top byte alone, as WRMASK says;
• nothing is written outside the 1280-wide screen (sentinel check at x=1280/2047);
• STATUS is idle afterwards, so the guest's wait_idle returns;
• no primitive is left mid-flight.

Evidence: this is the conformance test itself — it fails if the aux-plane base, the bounds check, the FIFO drain or the block-fill writeback regresses.

Tests: cargo test -p iris --lib --features rex-jit dev::ng1::rex3::tests::jit_tests::jit_full_screen_aux_clear_like_irix7 — 1 passed; the rest of the REX3 suite passes unchanged.