feat: fast markdown parser and comprehensive test coverage

[xiaolai]

Summary

• Fast parser implementation: New markdown-it based parser (~40-50x faster than remark) for simple markdown content
• Content-addressed parsing cache: LRU cache with FNV-1a hashing for large documents (≥5KB), with hit/miss tracking
• Async parsing with web worker support: Non-blocking parsing for improved UI responsiveness
• Comprehensive test coverage: 12 new test files, bringing total to 175 test files with 2594 tests passing

Key Changes

Performance

• fastParser.ts - Lightweight markdown-it parser for simple content (no math/wiki-links)
• parsingCache.ts - Content-addressed caching with LRU eviction (max 20 entries), hit rate tracking
• workerAdapter.ts - Web worker integration for background parsing
• markdownWorker.ts - Dedicated worker for async markdown parsing

Test Coverage

Module	Tests Added
Mermaid syntax detection	26
Terminal markdown (detector + ANSI renderer)	64
Keyboard shortcuts store	39
Date/path utilities	72
HTML sanitization/XSS prevention	69
Markdown pipeline (parser, adapter, cache, performance)	~100

Security

• Path traversal attack prevention tests
• XSS sanitization coverage for HTML, SVG, KaTeX

Design Decisions

Why remark as primary parser? Rich plugin ecosystem for:

• Math (LaTeX): $x^2$, $$E=mc^2$$
• Wiki links: [[page]]
• GFM: tables, task lists, strikethrough
• Custom syntax: highlight, underline, sub/superscript

Why markdown-it as fast path? ~40-50x faster for simple markdown. canUseFastParser() detects content requiring remark features and routes accordingly.

Conservative exclusions: Tables and strikethrough excluded from fast path due to MDAST output differences between parsers.

Test plan

• All 2594 tests pass (pnpm test)
• TypeScript compiles without errors (pnpm typecheck)
• ESLint passes (pnpm lint)
• Manual testing of editor performance with large documents