fix(tests): restore the dark-mode spec's light-mode reference PNG

tests/wizard-04-finish-headless.png is a spec-owned reference image the
dark-mode spec diffs against in-process (toolbar must ignore the
browser's dark theme) — it was never a promotable baseline, but the
baselines-to-R2 migration (6dfa6eb) swept it out of git with the rest of
the PNGs, and every e2e run since fails with ENOENT at
screenshot-compare.ts:56. Restored verbatim from 6dfa6eb^ and documented
as deliberately committed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/kicad/utils/screenshot-compare.ts

@@ -30,7 +30,13 @@ export type ReferenceComparison = {
     meanChannelDiff: number;
 };
 
-/** Post-wizard pcbnew baseline screenshot (committed) and its toolbar region. */
+/**
+ * Post-wizard pcbnew LIGHT-MODE reference screenshot and its toolbar region.
+ * Deliberately COMMITTED to git — this is a spec-owned reference image the
+ * dark-mode spec diffs against in-process, NOT a promotable baseline (it is
+ * not in the R2 manifest and must not be moved there; deleting it broke the
+ * spec with ENOENT once — see the baselines-to-R2 migration).
+ */
 export const PCBNEW_REFERENCE = path.resolve(__dirname, '../../wizard-04-finish-headless.png');
 export const PCBNEW_HEADER_REGION: ReferenceRegion = {
     // maxDiffRatio counts every pixel differing by >16 in any channel, so it is dominated