feat(transcription): generalize OnnxNpuTranscriber -> OnnxTranscriber with DirectML EP (P2)

load() now selects OpenVINO (NPU) or DirectML (AMD/Intel GPU) by BackendId over
the same ONNX artifacts; ensure_runtime_env takes the target runtime DLL. Adds
an opt-in directml_transcribes_speech spike for on-GPU validation.

src-tauri/src/transcription/npu.rs

@@ -1,12 +1,15 @@
-//! NPU transcriber: Whisper ONNX on the Intel NPU via ONNX Runtime + OpenVINO
-//! (T3.4, ADR-0004). Same `Transcriber` trait and `TranscriptSegment` output as
-//! the whisper.cpp path, so callers never branch on engine.
+//! ONNX transcriber: Whisper ONNX via ONNX Runtime, on either the Intel NPU
+//! (OpenVINO EP, T3.4) or a DX12 GPU (DirectML EP, P2 — the non-Vulkan path for
+//! AMD/Intel). One engine, EP chosen by the requested `BackendId`; same
+//! `Transcriber` trait and `TranscriptSegment` output as the whisper.cpp path,
+//! so callers never branch on engine. The ONNX model artifacts are identical
+//! across EPs (an EP is a device backend, not a different graph).
 //!
 //! Split of work, validated by the T3.4 spike (encoder ~3.6× faster on NPU):
-//! - **encoder** (fixed `[1,80,3000]` shape) runs on the **NPU** via OpenVINO —
-//!   the expensive graph, and the shape NPUs want;
+//! - **encoder** (fixed `[1,80,3000]` shape) runs on the **accelerator** (NPU or
+//!   GPU) — the expensive graph, and the fixed shape accelerators want;
 //! - **decoder** (dynamic, autoregressive) runs greedy on the **CPU** EP — a
-//!   dynamic KV loop is a poor NPU fit and the cheap half anyway.
+//!   dynamic KV loop is a poor accelerator fit and the cheap half anyway.
 //!
 //! ponytail: greedy, no-KV-cache decode (re-feeds the full token prefix each
 //! step). Correct and simple; windows are short so the token count is small.
@@ -14,7 +17,9 @@
 
 use super::{AudioWindow, SegmentSink, Transcriber, TrxError};
 use crate::models::{BackendId, TranscriptSegment};
-use ort::execution_providers::OpenVINOExecutionProvider;
+use ort::execution_providers::{
+    DirectMLExecutionProvider, ExecutionProviderDispatch, OpenVINOExecutionProvider,
+};
 use ort::session::Session;
 use ort::value::Tensor;
 use std::collections::HashMap;
@@ -25,7 +30,7 @@ use std::sync::Mutex;
 /// Safety cap on generated tokens per 30 s window (Whisper's own max is 448).
 const MAX_NEW_TOKENS: usize = 224;
 
-pub struct OnnxNpuTranscriber {
+pub struct OnnxTranscriber {
     encoder: Mutex<Session>,
     decoder: Mutex<Session>,
     enc_input: String,
@@ -44,15 +49,21 @@ pub struct OnnxNpuTranscriber {
     next_id: AtomicU64,
 }
 
-impl OnnxNpuTranscriber {
-    /// Points `ort` (load-dynamic) at the on-demand-downloaded runtime and puts
-    /// its DLLs on the search path, unless the caller already set `ORT_DYLIB_PATH`
-    /// (the test harness does, to target a dev runtime). Idempotent.
-    fn ensure_runtime_env() {
+impl OnnxTranscriber {
+    /// Points `ort` (load-dynamic) at the given on-demand-downloaded runtime DLL
+    /// and puts its sibling DLLs on the search path, unless the caller already
+    /// set `ORT_DYLIB_PATH` (the test harness does, to target a dev runtime).
+    /// Idempotent.
+    ///
+    /// `ORT_DYLIB_PATH` is process-global and `ort` dlopens it once, so a single
+    /// process run must not mix EPs from different runtime bundles (OpenVINO vs
+    /// DirectML). That's fine: `best()` picks one backend per session.
+    // ponytail: one runtime per process. If backend hot-swapping between NPU and
+    // DirectML in one run is ever needed, that's a re-init-ort problem to solve then.
+    fn ensure_runtime_env(dll: std::path::PathBuf) {
         if std::env::var_os("ORT_DYLIB_PATH").is_some() {
             return;
         }
-        let dll = crate::paths::npu_runtime_dll();
         std::env::set_var("ORT_DYLIB_PATH", &dll);
         if let Some(dir) = dll.parent() {
             let path = std::env::var_os("PATH").unwrap_or_default();
@@ -185,32 +196,52 @@ impl OnnxNpuTranscriber {
     }
 }
 
-impl Transcriber for OnnxNpuTranscriber {
-    /// `model` is the directory holding the ONNX artifacts (see `onnx_models`).
-    /// `backend` is expected to be `Npu`; a non-NPU value is rejected so the
-    /// dispatcher can fall back to whisper.cpp rather than us guessing.
+impl Transcriber for OnnxTranscriber {
+    /// `model` is the directory holding the ONNX artifacts (see `onnx_models`) —
+    /// the same artifacts for every EP. `backend` selects the accelerator:
+    /// `Npu` → OpenVINO EP; `Amd`/`Intel` → DirectML EP (the non-Vulkan GPU
+    /// path). Any other value is rejected so the dispatcher can fall back to
+    /// whisper.cpp rather than us guessing.
     fn load(model: &Path, backend: BackendId) -> Result<Self, TrxError> {
-        if backend != BackendId::Npu {
-            return Err(TrxError::Load(format!(
-                "OnnxNpuTranscriber only serves the NPU backend, got {backend:?}"
-            )));
-        }
-        Self::ensure_runtime_env();
+        // Pick the runtime bundle + encoder EP for the requested accelerator.
+        // error_on_failure makes a failed accelerator registration LOUD (Err)
+        // instead of a silent CPU fallback, so the dispatcher can cleanly drop
+        // to the whisper.cpp path (T3.4 spike lesson).
+        let (runtime_dll, encoder_ep): (std::path::PathBuf, ExecutionProviderDispatch) =
+            match backend {
+                BackendId::Npu => (
+                    crate::paths::npu_runtime_dll(),
+                    OpenVINOExecutionProvider::default()
+                        .with_device_type("NPU")
+                        .build()
+                        .error_on_failure(),
+                ),
+                // ponytail: DirectML device_id 0 = the default DX12 adapter.
+                // Map the DXGI adapter index here if a multi-GPU box ever needs
+                // to pin a specific AMD/Intel card.
+                BackendId::Amd | BackendId::Intel => (
+                    crate::paths::directml_runtime_dll(),
+                    DirectMLExecutionProvider::default()
+                        .with_device_id(0)
+                        .build()
+                        .error_on_failure(),
+                ),
+                other => {
+                    return Err(TrxError::Load(format!(
+                        "OnnxTranscriber serves NPU/AMD/Intel, got {other:?}"
+                    )))
+                }
+            };
+        Self::ensure_runtime_env(runtime_dll);
 
         let encoder_path = model.join("encoder_model.onnx");
         let decoder_path = model.join("decoder_model.onnx");
         let tokenizer_path = model.join("tokenizer.json");
         let gen_config_path = model.join("generation_config.json");
 
-        // Encoder pinned to the NPU. error_on_failure makes a failed NPU
-        // registration LOUD (Err) instead of a silent CPU fallback, so the
-        // dispatcher can cleanly drop to the whisper.cpp path (T3.4 spike lesson).
         let encoder = Session::builder()
             .map_err(|e| TrxError::Load(e.to_string()))?
-            .with_execution_providers([OpenVINOExecutionProvider::default()
-                .with_device_type("NPU")
-                .build()
-                .error_on_failure()])
+            .with_execution_providers([encoder_ep])
             .map_err(|e| TrxError::Load(e.to_string()))?
             .commit_from_file(&encoder_path)
             .map_err(|e| TrxError::Load(e.to_string()))?;
@@ -434,7 +465,7 @@ mod tests {
         };
         let wav = std::env::var("WA_NPU_TEST_WAV").expect("set WA_NPU_TEST_WAV");
         let t0 = std::time::Instant::now();
-        let t = OnnxNpuTranscriber::load(Path::new(&model_dir), BackendId::Npu)
+        let t = OnnxTranscriber::load(Path::new(&model_dir), BackendId::Npu)
             .expect("load NPU transcriber");
         let load_ms = t0.elapsed().as_millis();
         let t1 = std::time::Instant::now();
@@ -455,4 +486,45 @@ mod tests {
             );
         }
     }
+
+    /// Real DirectML GPU inference (P2), opt-in. Point `ORT_DYLIB_PATH` at a
+    /// DirectML-EP `onnxruntime.dll` (or stage `runtime\directml\`), reuse the
+    /// same ONNX model dir as the NPU spike, and run on the Intel/AMD GPU:
+    ///   ORT_DYLIB_PATH=…directml\onnxruntime.dll WA_DML_MODEL_DIR=… \
+    ///   WA_DML_TEST_WAV=… cargo test --release --features npu \
+    ///   directml_transcribes -- --ignored --nocapture
+    #[test]
+    #[ignore = "requires a DirectML runtime + model + wav; run manually on a GPU"]
+    fn directml_transcribes_speech() {
+        let Some(model_dir) = std::env::var_os("WA_DML_MODEL_DIR") else {
+            eprintln!("skip: set WA_DML_MODEL_DIR");
+            return;
+        };
+        let wav = std::env::var("WA_DML_TEST_WAV").expect("set WA_DML_TEST_WAV");
+        // Intel and AMD both take the DirectML EP; either BackendId exercises it.
+        let backend = match std::env::var("WA_DML_BACKEND").as_deref() {
+            Ok("amd") => BackendId::Amd,
+            _ => BackendId::Intel,
+        };
+        let t0 = std::time::Instant::now();
+        let t = OnnxTranscriber::load(Path::new(&model_dir), backend).expect("load DirectML");
+        let load_ms = t0.elapsed().as_millis();
+        let t1 = std::time::Instant::now();
+        let segs = t.transcribe_file(Path::new(&wav)).expect("transcribe");
+        let infer_ms = t1.elapsed().as_millis();
+        let text = segs
+            .iter()
+            .map(|s| s.text.as_str())
+            .collect::<Vec<_>>()
+            .join(" ")
+            .to_lowercase();
+        eprintln!("[spike] backend={backend:?} load={load_ms}ms infer={infer_ms}ms text={text:?}");
+        assert!(!text.trim().is_empty(), "transcript was empty");
+        if let Ok(expect) = std::env::var("WA_DML_EXPECT") {
+            assert!(
+                text.contains(&expect.to_lowercase()),
+                "transcript {text:?} missing expected {expect:?}"
+            );
+        }
+    }
 }