Minimal txtracker that records which peer delivered each transaction
and credits peers when their transactions appear on chain. Provides
the PeerInclusionStats needed by the dropper's protection logic.
Design:
- NotifyReceived(peer, txs): records deliverer per tx hash (called
from handler_eth.go when tx bodies arrive via P2P)
- Subscribes to ChainHeadEvent, fetches block txs, credits the
delivering peer for each included tx
- Per-peer EMA of recent inclusions (alpha=0.05), updated every block
- LRU eviction at 262K entries to bound memory
- Mutex-based (not channel-based) for simplicity — the hot path
(NotifyReceived) is a fast map insert
Wired into the dropper via an adapter callback in backend.go that
converts txtracker.PeerStats to the dropper's PeerInclusionStats.
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