[P2] Charge drivers round negative half-tie attribution differently across platforms #51

Summary

The Swift and Kotlin chargeDrivers twins round an exact negative half-tie contribution differently. For the same default-path input, the user-facing HRV driver reports deltaPoints = -1 on Swift and deltaPoints = 0 on Kotlin.

The headline Charge score is unchanged. The mismatch is in its driver attribution and can also affect exact row ordering when magnitudes tie.

Found by the Recovery charge-drivers differential rollout at base fd48e88872f70be232353429a63543febfa52052.

Minimal reproduction

Invoke the real seven-argument/default-arg8 path with:

{
  "hrv": 29.99117725828923,
  "rhr": 60.0,
  "hrvBaseline": {
    "baseline": 30.0,
    "spread": 0.55,
    "nValid": 14,
    "nightsSinceUpdate": 0,
    "status": "trusted"
  }
}

For the HRV marginal, full - neutralised is exactly -0.5 as a Double (0xbfe0000000000000).

Native observed behavior

All row fields except deltaPoints are identical:

Cause

The production helpers use different tie rules:

• Swift: Int((full - neutralised).rounded()), which rounds this tie away from zero.
• Kotlin: (full - neutralised).roundToInt(), which rounds this tie toward positive infinity.

Duplicate search

This is related to, but not duplicated by:

• [P2] Recovery trace rounds negative half-ties differently on Swift and Kotlin #38, which was limited to the two-decimal engineer-facing Recovery trace and is closed after its focused fix/regression fixture.
• [P2] Recovery trace rounding diverges for signed zero and large finite values #47, which covers signed zero and huge finite inputs in the trace rounding helper.

No open or closed issue was found for chargeDrivers, RecoveryDrivers, or deltaPoints negative-half-tie behavior.

Impact and severity

P2. The Charge score itself is unchanged, but its user-visible explanation is platform-dependent by one point and exact magnitude-based ordering can diverge.

Acceptance criteria

• Define one cross-platform integer rounding rule for charge-driver marginal points.
• Emit identical deltaPoints for positive and negative half-ties, including this exact -0.5 reproduction.
• Preserve ordinary non-tie contributions and the headline Charge score.
• Preserve exact ordered output across platforms when rounded magnitudes tie.
• Add mirrored unit tests plus an exact native differential fixture covering the true default-arg8 path.

No production fix is included in the rollout that discovered this issue.