fix(drift): treat compound extensions like .d.ts as not-paths

[dyk1454683243-sudo]

What

Widen EXTENSION_ONLY so compound file-extension tokens such as `.d.ts`, `.d.mts`, and `.d.cts` are treated as extension-only prose and never become path claims.

The previous pattern /^\.[A-Za-z0-9]+$/ matched .ts / .yaml but not extra dotted segments. Inline `.d.ts` still hit KNOWN_EXTENSIONS (ends with .ts) and was not filtered by isNotAPath, so the SDK prose example in #202 became MISSING_PATH twice.

Real paths with extensions stay claims: src/index.ts, .mex/ROUTER.md, .github/workflows/ci.yml.

Why

Addresses #202 (bullet 2 only).

Do not treat this as Fixes #202 or Closes #202 — that would auto-close the whole mega-issue.

Remaining #202 bullets stay open for follow-ups:

1. Symbol notation (src/auth/login.validateToken)
2. Hypothetical examples (./x, x.ts, x/index.ts)
3. Alternation shorthand (tests/grounding\\|traversal\\|…)
4. Scoped-package resolve (@scope/pkg/package.json vs bare specifier)
5. gitignore trailing-slash directory exemption

Type of change

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

How to test

1. npx vitest run src/drift
2. Confirm `.d.ts` / `.d.mts` / `.d.cts` produce zero path claims and zero MISSING_PATH.
3. Confirm existing MISSING_PATH false positives: negation only heading-scoped, non-path tokens treated as paths, dot-dir glob blind spot #143 cases stay green, and real extension paths (src/index.ts, .mex/ROUTER.md, .github/workflows/ci.yml) are still checked.
4. npm run typecheck

Checklist

• Focused drift tests pass (npx vitest run src/drift — 12 passed, 1 skipped #201 version-shaped case left untouched)
• Typecheck passes (npm run typecheck)
• Tests pass (npm test) — full suite is not the gate for this one-regex 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

This is a drift claim-extraction filter only. It does not change graph identity, extractors, or the other #202 bullets.