perf(markdown): cache conversion patterns and index task lines

[PathGao]

Summary

• Cache Markdown conversion regular expressions with LazyLock.
• Index Markdown source lines once before annotating task checkboxes.
• Add a 64-item nested and quoted task-list regression test.

Dependency

Depends on #345 because it optimizes the task-checkbox annotation protocol introduced there. It is intentionally separate from the preview-width, display-math, and checkbox-persistence portions of the current Stack.

Validation

• npm ci
• npm run check
• npm test (151 passing)