fix: excavate modules in canvas fallback

[nrps9909]

Summary

• render cellsToDraw in the Canvas fallback used when Path2D is unavailable
• preserve imageSettings.excavate behavior for older or limited browser environments
• add focused fallback coverage proving modules are not painted beneath an excavated image

Root cause

The Path2D branch generated its path from cellsToDraw, but the fillRect fallback iterated the original cells. An embedded transparent or semi-transparent image could therefore reveal dark QR modules underneath it whenever Path2D was unavailable.

Verification

• exact base: 7de25725d918f352d116abba5ebdece32b79e676
• regression before the fix: the focused fallback test expected only the background fill after a full-matrix excavation, but observed 335 fillRect calls
• npm test -- tests/fallback.test.tsx --runInBand — 1/1 passed after the fix
• npm test -- --runInBand — 2 suites, 9 tests passed
• npm run tsc — passed
• npm run lint — passed
• npm run compile — ESM, CJS, declarations, and Less build passed
• Prettier check and git diff --check — passed

AI assistance disclosure: Codex was used to trace the divergent Canvas rendering branches, construct the exact-base regression, implement the one-line fix, and draft this report. I verified the failure count and all listed checks locally.

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[coderabbitai]

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Run ID: c44e1c8a-0ec7-4847-9baa-47a2fc712518

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