calibrate-serve: 会话在 duration_s 到期时永远处于 `finalizing` 状态,而帧数小于 min_frames — 没有 API 逃逸

Three layers line up (paths/lines against v2051):

1. wifi-densepose-signal/src/ruvsense/calibration.rs:532 — BaselineRecorder::finalize() correctly returns Err(CalibrationError::InsufficientFrames { got, need }) when frame_count < config.min_frames.

2. wifi-densepose-cli/src/calibrate_api.rs:605 (async fn finalize) — writes the "finalizing" snapshot before calling the fallible recorder.finalize():