feat(cog-pose): wire pose_v1.safetensors into inference path

Replace silent stub fallback in InferenceEngine with a tracing-aware
loader:

- emit `tracing::info!` when pose_v1.safetensors loads into the candle
  backend, including the resolved on-disk path
- emit `tracing::warn!` (instead of silently falling through) when the
  configured weights file is missing or no path is set at all, so the
  appliance log surfaces "no model — running stub" instead of just
  emitting confidence=0 frames forever
- inline unit tests covering both branches: stub fallback shape +
  confidence, and a real-weights forward pass that asserts the output
  decomposes into 17 (x, y) pairs all finite and in [0, 1]

The forward pass itself already matched the training script
(Conv1d 56->64->128->128 with dilations [1, 2, 4], GlobalMeanPool,
Linear 128->256->34, sigmoid) and the tensor names in the
safetensors file (`enc.c1/c2/c3.weight|bias`, `head.fc1/fc2.weight|bias`)
line up with the VarBuilder prefixes — no architecture changes
required, only loader hygiene.

Confidence is the published validation PCK@50 (0.185); v0.0.1 doesn't
emit per-frame confidence and we're not fudging that here.

v2/crates/cog-pose-estimation/src/inference.rs

@@ -145,8 +145,10 @@ impl InferenceEngine {
     }
 
     /// Create an engine with a specific weights path (used by `--config`
-    /// in `cog-pose-estimation run`). If `weights_path` is `None`, the
-    /// stub fallback is used.
+    /// in `cog-pose-estimation run`). If `weights_path` is `None` or the
+    /// file does not exist on disk, the engine falls back to the
+    /// centred-skeleton stub and emits a `tracing::warn!` so the
+    /// appliance log shows why no real keypoints are coming through.
     pub fn with_weights(weights_path: Option<&Path>) -> Result<Self, Box<dyn std::error::Error>> {
         let device = pick_device();
         let inner = match weights_path {
@@ -159,9 +161,26 @@ impl InferenceEngine {
                     VarBuilder::from_mmaped_safetensors(&[p.to_path_buf()], DType::F32, &device)?
                 };
                 let net = PoseNet::new(vb)?;
+                tracing::info!(
+                    weights = %p.display(),
+                    "loaded pose_v1.safetensors into candle backend"
+                );
                 Some(Arc::new(LoadedModel { net }))
             }
-            _ => None,
+            Some(p) => {
+                tracing::warn!(
+                    weights = %p.display(),
+                    "pose weights file not found; falling back to centred-skeleton stub (confidence=0)"
+                );
+                None
+            }
+            None => {
+                tracing::warn!(
+                    "no pose weights path configured and no default weights found on disk; \
+                     falling back to centred-skeleton stub (confidence=0)"
+                );
+                None
+            }
         };
         Ok(Self { inner, device })
     }
@@ -255,3 +274,98 @@ fn default_weights_path() -> Option<std::path::PathBuf> {
     ];
     candidates.into_iter().find(|p| p.exists())
 }
+
+// ---------------------------------------------------------------------------
+// Unit tests — exercise the safetensors → forward-pass path. Integration-level
+// assertions (CLI surface, manifest round-trip, etc.) live in `tests/smoke.rs`.
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Locate `pose_v1.safetensors` from any of the cwds a `cargo test`
+    /// invocation might land in (workspace root, `v2/`, or the crate dir).
+    fn locate_weights() -> Option<std::path::PathBuf> {
+        let candidates = [
+            std::path::PathBuf::from("cog/artifacts/pose_v1.safetensors"),
+            std::path::PathBuf::from("crates/cog-pose-estimation/cog/artifacts/pose_v1.safetensors"),
+            std::path::PathBuf::from(
+                "v2/crates/cog-pose-estimation/cog/artifacts/pose_v1.safetensors",
+            ),
+        ];
+        candidates.into_iter().find(|p| p.exists())
+    }
+
+    #[test]
+    fn stub_fallback_when_weights_missing() {
+        // `with_weights(None)` must never panic and must produce a
+        // finite, well-shaped output so the runtime loop keeps making
+        // progress while the operator notices the warn log.
+        let engine = InferenceEngine::with_weights(None).expect("engine init");
+        assert_eq!(engine.backend(), "stub");
+        let out = engine.infer(&SyntheticInput.as_window()).expect("infer");
+        assert!(out.is_finite());
+        assert_eq!(out.keypoints.len(), OUTPUT_KEYPOINTS * 2);
+        assert_eq!(out.confidence, 0.0);
+    }
+
+    #[test]
+    fn weights_load_and_forward_produces_seventeen_keypoint_pairs() {
+        let Some(weights) = locate_weights() else {
+            eprintln!(
+                "(skipping — pose_v1.safetensors not on disk; run from the cog crate or repo root)"
+            );
+            return;
+        };
+        let engine = InferenceEngine::with_weights(Some(&weights)).expect("load real weights");
+        assert!(
+            engine.backend().starts_with("candle-"),
+            "expected candle backend, got {}",
+            engine.backend()
+        );
+
+        // Synthetic [56, 20] zero-input window — the documented "no-op"
+        // test signal. Anything finite and well-shaped proves the
+        // safetensors weights flowed through the forward pass.
+        let out = engine.infer(&SyntheticInput.as_window()).expect("infer");
+
+        // Shape: 17 (x, y) pairs = 34 scalars, no NaN, no Inf, all in
+        // sigmoid's [0, 1] range.
+        assert_eq!(
+            out.keypoints.len(),
+            OUTPUT_KEYPOINTS * 2,
+            "expected {} scalars for 17 keypoint pairs",
+            OUTPUT_KEYPOINTS * 2
+        );
+        let pairs: Vec<[f32; 2]> = out
+            .keypoints
+            .chunks_exact(2)
+            .map(|c| [c[0], c[1]])
+            .collect();
+        assert_eq!(pairs.len(), OUTPUT_KEYPOINTS, "expected 17 (x, y) pairs");
+        for (i, [x, y]) in pairs.iter().enumerate() {
+            assert!(
+                x.is_finite() && y.is_finite(),
+                "keypoint {i} not finite: ({x}, {y})"
+            );
+            assert!(!x.is_nan() && !y.is_nan(), "keypoint {i} is NaN");
+            assert!(
+                (0.0..=1.0).contains(x) && (0.0..=1.0).contains(y),
+                "keypoint {i} out of [0,1]: ({x}, {y})"
+            );
+        }
+
+        // Confidence is the published PCK@50 (constant for v0.0.1), so
+        // anything > 0 proves we didn't silently fall through to the stub.
+        assert!(out.confidence > 0.0);
+        assert!(out.confidence.is_finite());
+    }
+
+    #[test]
+    fn rejects_wrong_shape_input_before_any_forward_pass() {
+        let engine = InferenceEngine::with_weights(None).expect("engine init");
+        let bad = CsiWindow { data: vec![0.0; 7] };
+        assert!(engine.infer(&bad).is_err());
+    }
+}