docs(plans): add design for classroom local speech bridge support

docs/plans/2026-07-29-classroom-local-bridge-design.md

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+# Classroom: use the Local Speech Bridge when available
+
+## Problem
+
+`classroom.vue` always calls the cloud `/api/atc/say` endpoint for instructor
+TTS. `live-atc.vue` already prefers a locally running Speech Bridge
+(`app/composables/useLocalSpeechBridge.ts`, which probes `127.0.0.1:8765-8770`
+for `GET /health` and falls back to the cloud path on failure/timeout) via
+`useRadioSpeech.ts`. Classroom should get the same local-first behavior.
+
+## Scope
+
+- Classroom only performs TTS (`/api/atc/say`); it has no PTT/mic input, so
+  `/api/atc/ptt` is out of scope.
+- No UI indicator for bridge status — matches live-atc's silent fallback.
+
+## Change
+
+In `app/pages/classroom.vue`:
+
+1. Import `postWithLocalFallback, useLocalSpeechBridge` from
+   `~/composables/useLocalSpeechBridge` and call `useLocalSpeechBridge()` once
+   during setup (same as `useRadioSpeech.ts:86`).
+2. In `requestSayAudio()` (`classroom.vue:4323`), replace the direct
+   `api.post('/api/atc/say', payload)` call with
+   `postWithLocalFallback(localUrl('/api/atc/say'), payload, () => api.post('/api/atc/say', payload))`,
+   matching `useRadioSpeech.ts:274-278`.
+
+No payload/response shape changes are needed — classroom's payload is already
+compatible with what the local bridge and cloud endpoint both expect, and
+`sayCache`/`pendingSayRequests` caching is unaffected since it is keyed by
+content, not transport.