[P2] Recovery trace rounds negative half-ties differently on Swift and Kotlin #38

Summary

The byte-aligned Recovery diagnostic trace uses different rounding rules for negative half-ties. A skin-temperature term of exactly -0.125 is rendered as -0.13 by Swift and -0.12 by Kotlin.

The Charge score itself is unchanged; this is a P2 parity bug in diagnostic output.

Minimal reproduction

Call recoveryTrace with a usable HRV baseline and:

skinTempDev = 0.125

The skin-temperature scale is 1.0, so the traced term is exactly -0.125.

Observed behavior

Cause

Swift's default Double.rounded() handles this negative half-tie away from zero. Kotlin's Math.round handles it toward positive infinity.

Impact

Recovery traces are intended to be byte-aligned and fixture-testable across platforms. This creates false drift reports, although the Charge score itself is unchanged.

Acceptance criteria

• Choose and document one common trace-rounding rule.
• Emit identical strings for positive and negative half-ties, including ±0.125.
• Preserve non-tie trace values and the returned Charge score.
• Add an exact differential fixture for the complete trace.

This is fully reproducible with unit tests; no hardware testing is required.