Keep recently seen balloons tracked outside boundary

[MrARM]

Motivation

• Fix behavior where balloons that drift outside the detection area stop being tracked by continuing to process packets for serials that were recently seen.
• Keep the solution CPU and memory efficient by using an in-memory lookup (not Redis) with a very cheap per-packet check.
• Ensure entries age out automatically so memory stays bounded.

Description

• Add tracker.go which implements a lightweight thread-safe serialTracker with wasSeenRecently, markSeen, and removeStale methods and a cleanup goroutine started by startStaleSeenSerialsCleanup that runs hourly.
• Instantiate the tracker in balloony.go as seenSerials = newSerialTracker() and stamp every incoming packet with seenSerials.markSeen(pkt.Serial, pkt.TimeReceived).
• Change out-of-bounds filtering to allow processing of packets whose serials were seen recently by checking wasSeen := seenSerials.wasSeenRecently(pkt.Serial) before skipping; first-time out-of-bounds serials are still ignored.
• Start the hourly cleanup worker at startup with startStaleSeenSerialsCleanup(seenSerials) and keep the lookup in memory only (no Redis changes).

Testing

• Ran go test ./... which completed successfully (no test files in the module).

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Codex Task