test: fuzz parser and source-map invariants

[luojiyin1987]

Summary

• Add fast-check property tests for parser and source-map invariants.
• Generate normal and malformed Markdown from reusable atoms and constructs.
• Run 1000 cases with a fixed default seed.
• Support failure replay through FAST_CHECK_SEED and FAST_CHECK_PATH.
• Keep production parser code unchanged.

Properties

• parseMdWithSourceMap() preserves AST parity with parseMd().
• Every node position stays inside the source bounds.
• Every supported value code unit has a bounded source range.
• Value and URL mappings remain monotonic.
• getRaw() remains consistent with parser positions and whole ranges.
• Value and URL mutations still produce consistency errors.
• Autolink URL mappings remain unavailable.

Validation

• pnpm test: 24 suites and 1979 tests passed.
• The property test ran 1000 generated documents.
• pnpm exec eslint __tests__/source-map/fuzz.spec.ts passed.
• pnpm run test:package passed.

[luojiyin1987]

Lockfile audit complete. I restored pnpm-lock.yaml from master and ran:

corepack pnpm@11.27.1 add -D fast-check@4.10.2

pnpm 11.27.1 produced a byte-identical lockfile to this PR. The peer-context updates are resolver normalization. I kept the generated result and made no manual lockfile edits. A frozen install also passes.