refactor(source-map): extract recording extension

[luojiyin1987]

What changed

• Move the micromark recording extension into src/source-map/recording-extension.ts.
• Move its private protocol types and helper functions with it.
• Export only recordingExtension from the new module.
• Limit its state interface to segments and urlSourceSpans.

Why

The recording extension depends on undocumented parser handler contracts.

This module seam isolates that parser-internals risk from source-map orchestration and reconstruction algorithms.

Scope

• No handler rewrites.
• No CompileContext changes.
• No URL or inline-code extraction.
• No shared SourceSpan type.
• No parser algorithm changes.
• No public interface changes.

Verification

• The moved metadata types and implementation are byte-for-byte identical after export normalization.
• pnpm run prepublishOnly passed.
• All 14 test suites passed.
• All 1,956 tests passed.
• Type tests, ESLint, build, and package checks passed.
• pnpm run bench:source-map completed.

This is the final planned source-map extraction. Further extraction is out of scope.