perf(logs): normalize integer timestamps without digit lists

[djwhitt]

Summary

• Replace Integer.digits/1 |> Enum.count/1 timestamp-unit detection with numeric range guards.
• Preserve the existing 7-, 10-, 13-, 16-, and 19-digit conversion behavior.
• Use the same allocation-free helper for explicit event timestamps and all supported trace start-time aliases.
• Add conversion-boundary, generated parity, day-bucket, trace-alias, and invalid-explicit-timestamp coverage.

Stack

1. perf(logs): reuse clean BigQuery-safe payloads #3765 — Reuse clean BigQuery-safe payloads
2. perf(logs): transform bodies before constructing LogEvent #3767 — Transform bodies before constructing LogEvent
3. perf(logs): normalize integer timestamps without digit lists #3768 — Normalize integer timestamps without digit lists
4. perf(logs): fast-path common UTC ISO 8601 timestamps #3769 — Fast-path common UTC ISO 8601 timestamps