refactor(source-map): extract code segment reconstruction

[luojiyin1987]

What changed

• Move code block source-map reconstruction into src/source-map/code-segments.ts.
• Export only buildCodeSegments from the new module.
• Keep one buildCodeSegments(md, node) call in build-source-map.ts.

Why

Code block mapping is one independent algorithm domain.

This extraction gives it one module seam. It also reduces the responsibilities of build-source-map.ts.

Scope

• No algorithm changes.
• No performance changes.
• No test structure changes.
• No URL or inline-code extraction.
• No recordingExtension changes.
• No public interface changes.

Verification

• The moved 424-line implementation is byte-for-byte identical after import and 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 PR follows the event-local state work in #87, #88, and #89.

Closes #92