True Sphinx incrementality via mtime manipulation keyed to the cache-build SHA

[mmcky]

#92 Phase 3: true Sphinx incrementality via mtime manipulation keyed to the cache-build SHA — touch only the PR diff so Sphinx rebuilds only changed pages.

• mtime scheme keyed to the SHA the cache was built at
• Full rebuild on deletions/renames/_toc.yml/_config.yml

Design notes and rejected alternatives (single-page builds, Sphinx -j) in PROJECT-OPTIMIZE-PREVIEWS.md.