fix(docs): make the digest reference link build-safe

[charles-fineman]

Outcome

The VitePress documentation build succeeds while retaining a working link to the canonical digest reference script.

Context

Full validation for #412 found that npm run docs:build fails on current main with one dead link:

• docs/spec/file-migrations-api.md links to ../../kit/filemigrate/testdata/digest_reference.py
• the script exists in the repository, but VitePress rejects the link because it escapes the documentation source tree

The link was added with the canonical digest vectors in #466. This failure is unrelated to #412 and should not be bundled into its query-validation change.

Acceptance criteria

• The digest-reference documentation links to a reachable source without escaping the VitePress documentation tree.
• npm run docs:build succeeds from a clean checkout.
• The documented --check and --write commands remain accurate.