fix(vital_signs): use circular variance for wrapped phases

[akhilesharora]

The heartbeat feature in process_frame was computed as arithmetic mean + arithmetic variance on phases that come from atan2() and are wrapped to (-pi, pi]. Two phases that straddle +/-pi are physically aligned but the linear formula treats them as ~2*pi apart, so you get ~pi^2 (~9.87) instead of ~1e-6. That noise rides into the heart-rate FFT buffer every time a subcarrier's phase crosses the wrap.

Swapped the inline math for a standard circular variance (1 - R, where R is the mean resultant length). The value is bounded [0, 1] instead of [0, ~pi^2] now, but the heartbeat FFT cares about relative spectral peaks, not absolute scale.

Tests:

cargo test -p wifi-densepose-sensing-server --no-default-features --lib vital_signs
test result: ok. 15 passed; 0 failed

4 new tests: one drives process_frame end-to-end and inspects the heartbeat_buffer; the other three hit the helper with wraparound, opposite, and identical inputs.

Related to the jumpy vitals in #519 and the +/-15 bpm jitter mentioned in #485. #485 wraps VitalSignDetector with an external pipeline but keeps it as the low-confidence fallback - this fix is complementary.

Closes #593

[ruvnet]

Reviewed end-to-end:

• Math is correct: 1 - R where R = sqrt(sin_sum^2 + cos_sum^2) / n is the standard Mardia circular variance.
• Bounded to [0, 1] instead of [0, ~pi^2], which is fine because the heartbeat FFT cares about relative spectral peaks (confirmed in the PR description).
• The regression test test_phase_variance_handles_wraparound is particularly nice — it asserts both the new bound AND that the old buggy formula on the same input produces ~pi^2, which makes a regression to the linear computation explicitly fail.
• test_process_frame_handles_wrapped_phases is the end-to-end check that anyone can run against main to observe the bug and against this branch to observe the fix. Excellent reproducer.
• Identical-phase and opposite-phase edge cases covered.
• Scoped to a single file. Well-commented. Closes #593.

Approving. Merging with squash.