Instrument Swift checkout failures

[tiagocandido]

Summary

• Record Swift checkout errors, protocol decoding failures, navigation retry outcomes, and monotonic navigation duration
• Add a default-on telemetry setting with an explicit runtime opt-out that discards buffered measurements
• Prevent duplicate HTTP/navigation error metrics and align retry funnel semantics (started, failed, not_attempted)
• Lazily initialize the recorder and keep production transport out of unit tests
• Guard recorder creation against a concurrent runtime opt-out
• Document the bounded diagnostic data and explicit opt-out

Validation

• Swift formatting
• Full Swift unit test suite
• Swift API compatibility checks