feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/libs/markstream-vue/test/benchmark/README.md

@@ -0,0 +1,101 @@
+# Performance Testing Tools
+
+This directory contains tools and documentation for analyzing the performance of KaTeX Worker-based rendering.
+
+## Files
+
+### Benchmark Tests
+- **`katex-worker-vs-direct.test.ts`** - Comprehensive benchmark comparing Worker vs direct rendering
+- **`katex-performance-demo.html`** - Interactive visual demo for performance testing
+
+### Documentation
+- **`katex-worker-performance-analysis.md`** - Complete guide on measuring performance
+- **`docs/zh/katex-cache-analysis.md`** - Quick summary (Chinese)
+
+## Quick Start
+
+### Run Benchmarks
+
+```bash
+# Run all performance tests
+pnpm test test/benchmark/
+
+# Run specific test
+pnpm test test/benchmark/katex-worker-vs-direct.test.ts
+
+# With detailed output
+pnpm test test/benchmark/katex-worker-vs-direct.test.ts -- --reporter=verbose
+```
+
+### Visual Demo
+
+Open `katex-performance-demo.html` in a browser to see an interactive performance comparison.
+
+### Runtime Monitoring
+
+Add to your application:
+
+```typescript
+import { enablePerfMonitoring, getPerfReport } from 'markstream-vue/utils/performance-monitor'
+
+enablePerfMonitoring()
+
+// After some usage...
+setTimeout(() => getPerfReport(), 30000)
+```
+
+Or use browser console:
+
+```javascript
+window.__katexPerfReport()
+```
+
+## Key Insights
+
+### Cache is Critical
+- Cache hit: **~0.01ms**
+- Worker render: **5-50ms**
+- **Speedup: 500-5000x** 🚀
+
+### Real-World Performance
+- Typical cache hit rate: **70-85%**
+- Performance improvement: **3-10x**
+- Memory overhead: **~36KB for 200 formulas**
+
+### When Worker + Cache Wins
+✅ Complex formulas (>10ms render time)
+✅ Multiple formulas per page (>5)
+✅ Repeated formulas (cache hit rate >30%)
+✅ User interaction matters (scrolling, animations)
+
+## Decision Matrix
+
+| Scenario | Recommendation |
+|----------|----------------|
+| Complex formulas | ✅ Use Worker + Cache |
+| Many formulas per page | ✅ Use Worker + Cache |
+| High repetition | ✅✅ Cache is highly effective |
+| Simple formulas only | ⚠️ Worker overhead acceptable |
+| SSR/Node.js | ⚠️ Fallback to direct render |
+
+## Current Architecture (Best Practice)
+
+```typescript
+// 1. Check cache first (<0.01ms)
+const cached = cache.get(key)
+if (cached)
+  return cached
+
+// 2. Use Worker (non-blocking)
+return renderKaTeXInWorker(content)
+  .then((html) => {
+    cache.set(key, html) // Populate cache
+    return html
+  })
+  .catch(() => {
+    // 3. Fallback to direct render
+    return katex.renderToString(content)
+  })
+```
+
+**This hybrid approach is optimal!** ✅