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/packages/markstream-vue2/src/utils/mathRenderCache.ts

@@ -0,0 +1,39 @@
+interface CachedMathRender {
+  html: string
+}
+
+const MAX_CACHE_SIZE = 200
+const mathRenderCache = new Map<string, CachedMathRender>()
+
+function touchCacheEntry(key: string, value: CachedMathRender) {
+  mathRenderCache.delete(key)
+  mathRenderCache.set(key, value)
+  if (mathRenderCache.size > MAX_CACHE_SIZE) {
+    const oldestKey = mathRenderCache.keys().next().value
+    if (oldestKey)
+      mathRenderCache.delete(oldestKey)
+  }
+}
+
+export function getCachedMathRender(key: string | null | undefined) {
+  if (!key)
+    return null
+  const cached = mathRenderCache.get(key) || null
+  if (cached)
+    touchCacheEntry(key, cached)
+  return cached
+}
+
+export function setCachedMathRender(key: string | null | undefined, html: string) {
+  if (!key || !html)
+    return
+  touchCacheEntry(key, { html })
+}
+
+export function clearCachedMathRender(key?: string | null) {
+  if (!key) {
+    mathRenderCache.clear()
+    return
+  }
+  mathRenderCache.delete(key)
+}