[DJ Semantic Fingerprint 5] Document fingerprint compatibility and migrations

[philipfweiss]

Fingerprint consumers can store server-owned hashes in Git and use changes to select reviewers, but the stack does not yet document the stability boundary or upgrade procedure. Without that contract, a server or algorithm change could produce a repository-wide metadata diff and notify unrelated owners. This PR defines the compatibility and migration runbook before consumers depend on the hashes.

Depends on #2490.

This PR documents:

• the complete behavior covered by a fingerprint version
• the conditions required for hashes to remain stable
• how reflected and unresolved external tables affect fingerprints
• normal server upgrades without metadata churn
• dual-version algorithm migrations through a dedicated metadata pull request
• review fanout limits, migration mode, and unknown-state handling
• rollback steps and release checklists

Verification:

npx --yes markdownlint-cli2@0.4.0 content/0.1.0/docs/developers/semantic-fingerprint-versioning.md
Summary: 0 error(s)

Semantic fingerprint stack

This is PR 5 of 5:

1. #2482: Add semantic node fingerprints
2. #2488: Evaluate fingerprints across deployment graphs
3. #2483: Add semantic fingerprints to deployment impact
4. #2490: Bulk endpoint for viewing fingerprints and testing fixtures
5. #2534: Document fingerprint compatibility and migrations (this PR)