Implement LRU cache for storing hashes to filter out for flood repeating

[weebl2000]

Replace the FIFO (cyclic index) hash table with an LRU cache using uint32_t millis() timestamps. The oldest entry is evicted instead of the next sequential slot, so frequently-seen hashes stay in the table longer.

Changes:

• _next_idx replaced by _last_seen[MAX_PACKET_HASHES] (uint32_t timestamps)
• On duplicate hit: refresh timestamp (LRU touch)
• On insert: evict the entry with the oldest timestamp
• restoreFrom(): restored hashes get fresh timestamps; empty slots stay at 0
• saveTo(): writes a dummy _next_idx for backward-compatible file format
• No timeout-based eviction — purely LRU

Why this helps:

Some busy nodes see more than 128 packets before a flooded packet returns. With FIFO, a hash gets evicted after 128 new packets regardless of how recently it was seen. With LRU, actively-seen hashes stay in the table while stale ones get evicted first.

For example, the Utrecht repeater sees ~658 packets/20 min with 72 neighbors. With FIFO:

• Time to cycle 128 slots: 128 / 0.548 pkt/s ≈ 234 seconds
• Any packet returning after ~4 minutes won't be filtered → loop

With LRU, recently-seen hashes survive even under high packet volume, so only truly stale entries get evicted.

Memory cost: +384 bytes (uint32_t[128] replaces int _next_idx)

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Build firmware: Build from this branch

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Mirror of meshcore-dev#1380