Reconcile sync-version.js and docs/architecture/versioning.md contract conflict

[serenakeyitan]

Problem

Two load-bearing contracts disagree about when `assets/tree/VERSION` and `src/products/tree/VERSION` should bump:

• `scripts/sync-version.js` enforces: `package.json.version === assets/tree/VERSION === src/products/tree/VERSION` on every `release:check`.
• `docs/architecture/versioning.md` says: `assets/tree/VERSION` bumps only on `assets/tree/` changes; `src/products/tree/VERSION` bumps only on `tree` CLI subcommand changes.

Every shipped release (#273, #275, #317, #323) has followed the script, not the doc.

Decide

Either:

• Relax `sync-version.js` to allow independent tree VERSIONs (then the doc becomes truth).
• Update `versioning.md` to say tree VERSIONs track package version (then the script becomes truth).

Surfaced on #323 during review.