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/playground-stream-presets.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest'
+import { findMatchingStreamPreset, getStreamPreset } from '../playground/src/composables/streamPresets'
+
+describe('playground stream presets', () => {
+  it('matches the SSE preset by exact control values', () => {
+    expect(findMatchingStreamPreset({
+      chunkDelayMin: 18,
+      chunkDelayMax: 44,
+      chunkSizeMin: 1,
+      chunkSizeMax: 4,
+      burstiness: 18,
+    })?.id).toBe('sse')
+  })
+
+  it('returns null when current controls no longer match a preset', () => {
+    expect(findMatchingStreamPreset({
+      chunkDelayMin: 18,
+      chunkDelayMax: 45,
+      chunkSizeMin: 1,
+      chunkSizeMax: 4,
+      burstiness: 18,
+    })).toBeNull()
+  })
+
+  it('resolves named presets and ignores custom ids', () => {
+    expect(getStreamPreset('proxy-buffered')?.label).toBe('Proxy Buffered')
+    expect(getStreamPreset('custom')).toBeNull()
+  })
+})