fix(drift): do not treat version-shaped refs as missing paths

[dyk1454683243-sudo]

What

Tighten the isUnrootedReference extension guard so a trailing numeric-only segment is not treated as a file extension.

release/2.1.0 used to match /\.[A-Za-z0-9]+$/ on .0, skip the prose/unrooted check, and report MISSING_PATH. The guard now requires an alphabetic character in that suffix, so git-flow / pinned-runtime tokens (release/2.1.0, python/3.11, node/20.11) reach the existing first-segment check.

Unskips does not claim a version-shaped branch name and adds a small regression that docs/foo.bar still stays a real path claim.

The narrower api/v1.2 case (first segment exists on disk) is unchanged.

Why

Fixes #201

mex check was flagging branch names and pinned runtimes as missing files whenever the last dotted segment was digits-only.

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. npx vitest run src/drift/__tests__/path-false-positives.test.ts
2. Confirm the previously skipped does not claim a version-shaped branch name case is active and green.
3. Existing MISSING_PATH false positives: negation only heading-scoped, non-path tokens treated as paths, dot-dir glob blind spot #143 path-false-positive cases should stay green.
4. npm run typecheck

Checklist

• Focused drift path tests pass (npx vitest run src/drift — 12/12)
• Typecheck passes (npm run typecheck)
• Full npm test — not used as a gate here; this environment’s Node 22.14.0 SQLite is built without FTS5, which fails graph/wiki suites unrelated to this change
• No breaking changes (or documented below)
• Tested locally with a real project

Code-graph changes

• This PR targets main
• A linked issue agrees on the bounded extractor/resolver scope
• The change follows the frozen LanguageExtractor or FrameworkResolver interface
• A focused fixture and assertions for the expected node/edge shape are included
• Any new grammar WASM, extension mapping, extractor, or resolver is registered
• No graph identity, reconciliation, schema, or drift-semantics changes are included, or a core / discuss-first issue is linked above