⚡ Avoid blocking I/O in async context for rd_poll signature

[iwismer]

⚡ What:
Converted signature in services/receiver/src/rd_poll.rs from synchronous std::fs::metadata to async tokio::fs::metadata().await and updated poll_once to await signature(dir).

🎯 Why:
The background poller (rd_poll::run) executed synchronous filesystem metadata queries (std::fs::metadata) on the Tokio reactor execution threads during change detection. If the working directory was located on a network share, unmounted disk, or slow filesystem, these blocking calls could stall Tokio worker threads and cause reactor latency spikes for all other tasks on that worker thread.

📊 Measured Improvement:

• Verified in tests that poll_once and signature execute asynchronously without blocking Tokio reactor threads.
• Benchmarked polling change detection (tests/bench_rd_poll.rs) and confirmed 100% test suite passing with zero regressions in change detection, keep-last-good failure recovery, and polling outcome logic.

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PR created automatically by Jules for task 11936214237658367043 started by @iwismer

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