Better tests and more user feedback while doing transcription; WIP.

Decisions.md

@@ -31,7 +31,7 @@ thread would freeze the UI. Communication via a small typed postMessage protocol
 (`src/worker/protocol.ts`) shared between UI and worker.
 
 ## D-004 — WebGPU with automatic WASM fallback
-**Status:** Accepted
+**Status:** Accepted (amended by D-017 — "auto" now probes WebNN first)
 Detect via `navigator.gpu?.requestAdapter()` inside the worker; pass
 `device: "webgpu" | "wasm"` to the pipeline. The active backend is surfaced in the UI
 but users don't have to configure anything. Encoder-heavy Whisper benefits ~10-40× from
@@ -222,3 +222,20 @@ latency); MediaRecorder + re-decoding compressed blobs (adds codec latency and
 device — the UI must keep tentative text visibly distinct from committed text.
 Server-engine live input (WebSocket streaming to faster-whisper) is a separate
 future decision; D-011's "WebSocket if needed" note is the hook.
+
+## D-017 — "Auto" device order: WebNN → WebGPU → WASM
+**Status:** Accepted (2026-07-14; amends D-004, which only knew WebGPU → WASM)
+With WebNN support added (commit "WebNN support initial"), the `"auto"` device
+preference resolves in priority order **WebNN → WebGPU → WASM**: the worker
+probes `navigator.ml` (WebNN) first, then `navigator.gpu.requestAdapter()`
+(WebGPU), and always keeps WASM as the last resort. The probe/ordering policy
+is pure and unit-tested (`deviceAttempts` in `src/worker/protocol.ts`,
+`tests/protocol.test.ts`); the worker only supplies the capability probe.
+Pipeline-creation failures walk the same chain — an auto-detected WebNN that
+fails still tries WebGPU before WASM. An **explicit** `"webnn"` or `"webgpu"`
+preference deliberately falls back straight to WASM (no detour through the
+other accelerator — the user asked for a specific backend, so honor it or take
+the universal fallback). The selected backend stays user-visible: the worker's
+`{status:"ready", device}` message drives a "Running on …" badge next to the
+Backend select (`ModelPicker`) and the engine line in the footer (D-004's
+"surfaced in the UI" requirement).