[P2] Recovery trace rounding diverges for signed zero and large finite values #47

Summary

The Swift and Kotlin Recovery trace rounding helpers still produce byte-different values for near-zero negatives and for large finite inputs.

This was found while reviewing the negative half-tie fix in #38. The Charge score is unaffected; only diagnostic trace formatting differs.

Minimal reproductions

Cause

Swift performs floating-point rounding and preserves negative zero. Kotlin currently routes through Math.round, which returns a Long:

• near-zero negatives become positive zero after division;
• large finite values saturate at the signed-64 limits before division.

Impact

Recovery trace output is intended to be exactly comparable across platforms. The signed-zero case is reachable for small driver terms. The large-value case is not physiologically plausible, but the public trace API has no documented finite bounds.

Severity: P2 because the Recovery score is unchanged.

Acceptance criteria

• Define one two-decimal trace-rounding contract for finite inputs.
• Normalize signed zero identically.
• Avoid platform-specific integer saturation, or explicitly reject/document inputs outside the supported range.
• Add mirrored tests for -0.004, +0.004, ±1e20, ordinary non-ties and positive/negative half-ties.
• Add an exact differential trace fixture in the Recovery rollout.

No hardware testing is required.