[P2] Charge driver skin-temperature text is locale-dependent across platforms #52

Summary

The Swift and Kotlin chargeDrivers twins render the same negative skin-temperature deviation differently. A native seeded case with skinTempDev = -0.35 produced -0.3 C vs baseline on Swift and -0.4 C vs baseline on Kotlin.

The Recovery charge-drivers contract requires stable en_US_POSIX/US formatting for exact user-facing rows. The headline Charge score is unchanged.

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

Reproduction

The native seeded case used:

{
  "hrv": 46.0,
  "rhr": 58.0,
  "resp": 14.0,
  "hrvBaseline": {
    "baseline": 51.0,
    "spread": 5.0,
    "nValid": 16,
    "nightsSinceUpdate": 1,
    "status": "trusted"
  },
  "rhrBaseline": {
    "baseline": 58.0,
    "spread": 4.0,
    "nValid": 12,
    "nightsSinceUpdate": 1,
    "status": "trusted"
  },
  "respBaseline": {
    "baseline": 15.0,
    "spread": 1.5,
    "nValid": 12,
    "nightsSinceUpdate": 1,
    "status": "trusted"
  },
  "sleepPerf": 0.8999999999999999,
  "skinTempDev": -0.35
}

Native observed behavior

The Swift native rollout stopped with actual="-0.3 C vs baseline" expected="-0.4 C vs baseline"; the Kotlin native output completed with -0.4 C vs baseline.

Cause

Swift's production skinTempDevText uses String(format: "%.1f", dev) without an explicit locale. Kotlin uses String.format(Locale.US, "%+.1f C vs baseline", skinTempDev). The same finite input therefore crosses different locale/formatting paths.

Duplicate search

This is related to the rounding family in #38 and #47, but neither issue covers the user-facing chargeDrivers skin-temperature formatter or its one-decimal locale contract. No exact open or closed duplicate was found.

Impact and severity

P2. The score is unchanged, but a user-visible driver value is byte-different across platforms and depends on the Swift runtime locale/formatter path.

Acceptance criteria

• Make the production Swift and Kotlin skin-temperature formatter use one explicit POSIX/US locale contract.
• Emit identical text for -0.35, +0.35, ordinary non-ties, and signed values around zero.
• Preserve the explicit sign, one-decimal precision, unit, and vs baseline suffix.
• Confirm respiration formatting remains byte-identical under the same locale contract.
• Add mirrored unit tests and exact native differential fixtures.

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