ci: make the DiagROM boot smoke actually run

[LinuxJedi]

Problem

The diagrom-smoke CI job has never executed, for three stacked reasons:

1. Never enabled: it was gated on vars.DIAGROM_URL, a repository variable that was never set, so the job silently skipped on every run.
2. Fetch was wrong anyway: the step saved whatever the URL serves directly as diagrom.rom, but diagrom.com only serves zip archives (there is no raw ROM URL upstream, and the GitHub repo holds source only).
3. Boot assumption went stale: the run step relied on the old ./diagrom.rom default ROM lookup, which was removed when the bundled AROS ROM became the no-argument fallback. Even a correctly fetched ROM would not have been booted.

The local ignored test diagrom_menu_preserves_left_margin_text_columns was broken by the same removed default, and its 6s screenshot expectation predates the current default machine (512K chip + 512K slow), where DiagROM's memory detection plus serial-mode countdown still own the screen at that point.

Changes

.github/workflows/ci.yml -- the job now runs unconditionally:

• Downloads the official stable V1.3 archive, checksum-pinned (frozen upstream since April 2023) and cached via actions/cache, so the author's site is hit roughly once rather than per-run. DIAGROM_URL remains as an override should the official URL move. The ROM is deliberately not committed: the author keeps distribution to the official site.
• Extracts DiagROM/DiagROM, the emulator-usable image per the archive's Readme, and boots it as an explicit positional ROM.
• Asserts on DiagROM's version banner (Amiga DiagROM V1.3 ...) arriving over the emulated serial port on stdout -- this proves the ROM's boot code executed, where the previous test -s diag.png passed even for a black screen. The serial log is uploaded alongside the screenshot artifact.
• Then runs the repaired asset-gated image-regression test against the fetched ROM.

tests/image_regression.rs -- the ignored test now passes the ROM positionally and screenshots the main menu at 35s (reached at ~28s on the default machine, then static while waiting for input), asserting the yellow Serial: status-bar label at the left margin of the full-overscan frame (230 pure-yellow pixels measured; threshold 100).

Verification

All steps were run locally end-to-end:

• Fetch/checksum/extract logic verified verbatim (zip sha256 a575f8fa..., extracted ROM 1803bfff...).
• Boot smoke: banner lands on stdout within the 6s window, screenshot non-empty.
• cargo test --release --locked --test image_regression diagrom -- --ignored passes (menu screenshot pixel-identical at 30s and 40s).
• cargo fmt --check and cargo clippy --release --tests clean.