Prevent stale translated chunks after source changes

[cursor]

Summary

• Reproduced the stale-output integrity bug where refreshed manifest.json could bless a changed chunk0001.md while old output_chunk0001.md was still merged into output.md.
• Fix create_manifest() to compare the previous manifest's per-chunk source hash before writing the refreshed manifest, removing stale output_chunk*.md and matching output_chunk*.meta.json when the source chunk changed.
• Add regression coverage that stale outputs are removed and merge refuses to proceed/delete stale output.md.

Runtime evidence

• Pre-fix minimal repro: validate_ok=true, merge_ok=true, final output.md contained OLD translated content that should not survive.
• Post-fix minimal repro: stale output removed, validate_ok=false, merge_ok=false, output_md_exists=false.

Tests

• python3 -m unittest tests.test_merge_and_build -v
• python3 -m unittest discover -s tests -p 'test_*.py' -v (207 tests, 3 skipped)
• python3 -m compileall scripts tests

Note: baseline Alice full-pipeline smoke could not be run in this checkout because tests/baselines/standard-alice/standard-alice.epub is absent (only SOURCE.md exists).