Add a stale-pattern drift checker

[theDakshJaitly]

Context

Add a checker that flags pattern files in patterns/ that aren't referenced from ROUTER.md or anything in context/ — orphans in the navigation graph.

⚠️ Read src/drift/checkers/index-sync.ts first. This partially overlaps with index-sync, and the goal is not to duplicate it. index-sync is about the index ↔ files relationship; this checker is about pattern files that exist but nothing links to them. Your PR should state clearly how the two differ.

Scope

• Flag pattern files with no inbound reference from ROUTER.md or context/*.md.
• Warning-level severity (an orphaned pattern is a smell, not necessarily an error).

Where

• New checker: src/drift/checkers/stale-pattern.ts — structural checker (operates on projectRoot, scaffoldRoot), follow the shape of index-sync.ts.
• Register in src/drift/index.ts (import + invoke in the structural block + push to checkerIssueCounts).
• Tests: test/checkers.test.ts.

Acceptance criteria

• Flags an orphaned pattern file; does not flag a referenced one.
• PR description explains how this differs from index-sync.
• Registered + counted in src/drift/index.ts.
• npm run typecheck && npm test && npm run build pass.

New to mex? Good first issue — 9 existing checkers to copy from. See CONTRIBUTING.md.