Float16Array for fp16 tensors; per-session WebGPU fallback (decoders stay on webgpu)

mega-asr.js

@@ -116,8 +116,8 @@ async function fetchWithCache(url, label, onProgress) {
 }
 
 // ---- ONNX session creation -------------------------------------------------
-// Build EP list: prefer requested device, always include WASM as fallback so
-// that "backend not found" / device init failures don't abort the whole load.
+// Always prefer the user-selected device; fall back to WASM only for the
+// session that fails (per-session, not global). Don't mutate state.device.
 function epList() {
   return state.device === "webgpu" ? ["webgpu", "wasm"] : ["wasm"];
 }
@@ -130,8 +130,7 @@ async function createSessionSimple(graphUrl, label, onProgress) {
     return sess;
   } catch (e) {
     if (state.device === "webgpu") {
-      log(`webgpu failed for ${label} (${e.message}); retrying with wasm`);
-      state.device = "wasm";
+      log(`webgpu failed for ${label} (${e.message}); retrying this session with wasm`);
       const sess = await ort.InferenceSession.create(graph, { executionProviders: ["wasm"] });
       log(`session ready: ${label} (wasm fallback)`);
       return sess;
@@ -152,8 +151,7 @@ async function createSession(graphUrl, dataUrl, label, onProgress) {
     return sess;
   } catch (e) {
     if (state.device === "webgpu") {
-      log(`webgpu failed for ${label} (${e.message}); retrying with wasm`);
-      state.device = "wasm";
+      log(`webgpu failed for ${label} (${e.message}); retrying this session with wasm`);
       const sess = await ort.InferenceSession.create(graph, {
         executionProviders: ["wasm"], externalData: externalFiles,
       });
@@ -451,7 +449,17 @@ function f32ToF16Bits(v) {
   return (sign << 15) | (newExp << 10) | (frac >> 13);
 }
 
+// onnxruntime-web 1.20+ wants fp16 data as Float16Array (Chrome 134+); fall
+// back to Uint16Array bit-pattern path on older engines (ORT will treat it
+// as raw fp16 bytes).
+const HAS_F16 = typeof Float16Array !== "undefined";
+
 function floatArrayToFp16(arr) {
+  if (HAS_F16) {
+    const out = new Float16Array(arr.length);
+    for (let i = 0; i < arr.length; i++) out[i] = arr[i];
+    return out;
+  }
   const u16 = new Uint16Array(arr.length);
   for (let i = 0; i < arr.length; i++) u16[i] = f32ToF16Bits(arr[i]);
   return u16;