[P2] Raw recovery overload can score without the required HRV baseline #40

Summary

The public recovery overload that accepts optional driver baselines documents the HRV baseline as required. However, hrvBaseline may be absent while the separate hrvBaselineUsable flag defaults to true.

Another optional term can therefore produce a Charge score without any HRV term.

Minimal reproduction

hrv = 50
rhr = 60
resp = nil
hrvBaseline = nil
rhrBaseline = nil
respBaseline = nil
sleepPerf = 0.85
hrvBaselineUsable = default (true)

Observed behavior

Both Swift and Kotlin return approximately 57.932425214874954. Under the documented contract the result should be nil/null.

Cause

The cold-start guard checks only hrvBaselineUsable; it does not require hrvBaseline. The missing HRV term is skipped and sleepPerf becomes the only weighted term.

Impact

A public caller can produce a plausible Charge score without the model's dominant required driver. Known production paths use the stricter baseline-state overload, so this is P2 rather than P1.

Acceptance criteria

• Return nil/null whenever the required HRV baseline is absent, or make absence unrepresentable.
• Preserve cold-start behavior for present but unusable baselines.
• Preserve scores for valid usable baselines.
• Add paired tests with each optional driver present while HRV baseline is absent.

This is pure analytics behavior; no hardware testing is required.