fix: harden backup cancellation and export state

[AJV20]

Summary

• Tracks backup/restore cancellation by operation ID so cancelled primaries cannot fall through to fallback or surface as failures.
• Adds syslog runtime fallback from pymobiledevice3 to idevicesyslog while preserving Stop/stale-stream safety.
• Guards detached Messages export completions with export identity + backup path, and strengthens regression coverage.

Verification

• Scripts/regression-tests.py — PASS 38 checks
• swift build — passed
• swift build -c release — passed
• bash Scripts/build.sh — passed
• Scripts/benchmark-performance.sh — passed (tree speedup ~5.86x, polling speedup ~1.55x)

Note: intentionally no version bump in this follow-up.