Let the probe's clock be its proof, not disable_cpu_ep_fallback

The strict flag refused the Hexagon over the ten quantise/dequantise
nodes at the graph's edges that QNN declines by policy, which cost
microseconds. A provider that hands real work to the CPU is slower than
the CPU floor and the timing already rejects it; the tablet measured
2.3 ms on the NPU against a 29.7 ms floor.

core/dr-inference-engine/src/engines.rs

@@ -89,7 +89,7 @@ pub fn run() {
         }
         log::info!("inference: compiling {name} for {}", rung.label());
         let started = std::time::Instant::now();
-        match crate::session::build(rung, role, &bytes, &cfg, false) {
+        match crate::session::build(rung, role, &bytes, &cfg) {
             Ok(session) => {
                 drop(session);
                 let mut s = state().lock().unwrap();

core/dr-inference-engine/src/lib.rs

@@ -253,7 +253,7 @@ fn acquire(role: Role, form: Form, bytes: &Arc<[u8]>) -> Result<Acquired, Error>
 
     // Built outside the registry lock: a TensorRT engine load is long enough
     // that another role's acquire should not wait on it.
-    let session = session::build(rung, role, bytes, &cfg, false)?;
+    let session = session::build(rung, role, bytes, &cfg)?;
     log::debug!("inference: {role:?} loaded on {}", rung.label());
     let entry = Arc::new(Loaded {
         rung,

core/dr-inference-engine/src/probe.rs

@@ -1,9 +1,9 @@
 //! Walk the ladder, once, by building real sessions (docs/inference.md §4).
 //!
-//! A rung is taken when a strict session builds on it, runs, and is faster
-//! than the floor. Both halves matter: a provider can register and then fail
-//! at partition time, and a provider can take a graph and run it slower than
-//! the CPU would have. The outcome is cached against a fingerprint of the
+//! A rung is taken when a session builds on it, runs, and is faster than
+//! the floor. Both halves matter: a provider can register and then fail at
+//! partition time, and a provider can take a graph — or quietly hand most
+//! of it back to the CPU — and run it slower than the CPU would have. The outcome is cached against a fingerprint of the
 //! runtime, the driver, the hardware and the models, and trusted until any
 //! of those changes.
 
@@ -135,9 +135,9 @@ fn probe_model(cfg: &Config) -> Option<(Role, PathBuf)> {
     smallest(Some(Role::Detector)).or_else(|| smallest(None))
 }
 
-/// Build strictly, run once for the engine, then time three runs; the
-/// median in milliseconds and, for a compiling rung, the cache key of the
-/// engine this just built.
+/// Build, run once for the engine, then time three runs; the median in
+/// milliseconds and, for a compiling rung, the cache key of the engine this
+/// just built.
 fn time_rung(
     rung: Rung,
     role: Role,
@@ -157,8 +157,8 @@ fn time_rung(
     };
     let bytes = std::fs::read(&path).map_err(|e| e.to_string())?;
     let started = Instant::now();
-    let mut session = crate::session::build(rung, role, &bytes, cfg, true)
-        .map_err(|e| first_line(&e.to_string()))?;
+    let mut session =
+        crate::session::build(rung, role, &bytes, cfg).map_err(|e| first_line(&e.to_string()))?;
     log::info!(
         "inference: {} session built in {:.1} s",
         rung.label(),

core/dr-inference-engine/src/session.rs

@@ -7,22 +7,16 @@ use crate::{Config, Role, Rung};
 
 /// Build a session for `bytes` on `rung`.
 ///
-/// `strict` is the probe's flag: with it, a provider that would hand any
-/// node to the CPU fails the build instead, so "the session built" means
-/// "the provider took the graph" and not "the provider registered" (§4).
-pub fn build(
-    rung: Rung,
-    role: Role,
-    bytes: &[u8],
-    cfg: &Config,
-    strict: bool,
-) -> ort::Result<Session> {
+/// Not strict about the CPU: `session.disable_cpu_ep_fallback` was tried as
+/// the probe's proof that a provider took the graph, and it refuses the
+/// Hexagon over the ten quantise/dequantise nodes at the graph's edges that
+/// QNN declines by policy and that cost microseconds. The probe's proof is
+/// its clock instead (§4): a provider that hands real work to the CPU is
+/// slower than the CPU floor and rejected by the same measurement.
+pub fn build(rung: Rung, role: Role, bytes: &[u8], cfg: &Config) -> ort::Result<Session> {
     let mut b = Session::builder()?
         .with_optimization_level(GraphOptimizationLevel::Level3)?
         .with_intra_threads(threads(cfg))?;
-    if strict && rung != Rung::Cpu {
-        b = b.with_config_entry("session.disable_cpu_ep_fallback", "1")?;
-    }
     // A Hexagon session loads the compiled context when there is one and
     // compiles it from the model when there is not; the engine thread is
     // what makes the second case rare (§6).

docs/inference.md

@@ -195,11 +195,14 @@ state where the provider registered and the session then failed, and a provider
 took the graph, and rejected every node at partition time. The probe therefore:
 
 1. Loads the runtime library (§3), or falls to tract and stops.
-2. For each rung in this platform's ladder, in order: builds a session for the **smallest model
-   in the set** (`scrfd_500m`) on that provider with `error_on_failure`, runs it once on a fixed
-   input, and reads back the provider assignment from the session — the rung is taken only if the
-   provider ran **at least 95% of the graph's nodes**. A provider that silently hands the graph to
-   the CPU is the CPU rung with extra overhead, and the app should say "CPU".
+2. Times the **smallest detector** on the CPU provider first — the floor. Then, for each rung
+   in this platform's ladder, in order: builds a session for the same model on that provider
+   with `error_on_failure`, runs it once on a fixed input, and times three more runs. **The rung
+   is taken only if its median beats the floor.** That one measurement is the proof the provider
+   took the graph: one that silently hands the work to the CPU is the CPU rung with extra
+   overhead, slower than the floor, and rejected. (ONNX Runtime's
+   `session.disable_cpu_ep_fallback` was the first draft of this proof and refuses the Hexagon
+   over the ten quantise/dequantise nodes at the graph's edges that QNN declines by policy.)
 3. Records the outcome — rung, runtime version, provider version, device identity (GPU name and
    compute capability; SoC model and Hexagon arch), and the models' content hashes — to a small
    file beside `shared_face_models_dir`. The next start-up trusts the file **unless** any of those