Parse fractional Retry-After values #43

Problem

RetryAfterParser uses toLongOrNull (RetryAfterParser.kt:193), so a fractional Retry-After: 1.5 silently falls through.

Proposed change

Parse with toDoubleOrNull (keep the existing negative/NaN/365-day clamps).

Acceptance

• Fractional seconds honoured
• Existing clamps intact
• Test

Priority: low · Effort: small