Docs sync + decision log

[flyingrobots]

Link spec from snapshot docs; add entry to decision-log.md.

[flyingrobots]

Closing: Spec links and decision log entries landed across recent PRs (e.g., PR-02 #93 BLAKE3 checklist/spec refs; PR-05 #96 docs rollup; PR-10 #101 docs notes; hotfix PR #102). Snapshot/commit spec is linked and Decision Log is current.