fix: emit nothing for a proof-less vote and drop blocks the stage already holds

[adust09]

Summary

Two defects found in the three-node devnet5 run behind #42, both now fixed with the failing case pinned by a test.

#43 — a vote with no proof no longer becomes an empty attestation

seed_block_votes files every vote a block carries into the counted pool with an empty proof set, on purpose (a block's merged proof is never split back per vote; leanSpec does the same). leanSpec's build_block emits one attestation per selected proof, so such a vote is marked processed and contributes nothing. Verity's select_votes instead pushed every eligible vote into the body, giving one attestation with no voters and an empty component list; the proposer's fold then handed leanVM nothing to aggregate and the slot's proposal was lost:

WARN block proof could not be built slot=39 error=cannot aggregate: aggregation failed: Nothing to aggregate: aggregated public keys is empty

select_votes now keeps a processed set apart from the selected groups: a vote whose coverage selection is empty is done with, counts against the distinct-vote budget, and emits nothing — leanSpec's shape. Two tests cover it: the proof-less vote alone, and a proof-less vote beside one that has a proof.

#45 — the verification stage drops what it already holds

The stage verified, and the chain task re-committed, every copy of a block that reached it: gossip plus a sync fetch, or two peers answering the same request. In the run, slot=25 and slot=37 were imported four times each (~90 ms of proof verification and a 230 KB proof write per copy). Now:

• a decoded block whose root the current snapshot holds is dropped before the registry lookup;
• an item identical to one already parked is not parked again (the first copy raised the gap);
• chain.rs::import returns before the commit for a root the store already has — the window between the stage's snapshot and the import's own;
• StageCounters::duplicates counts both, for verity-metrics to pick up.

Two stage tests exercise the first two paths over a genesis snapshot. The peer-churn half of #45 (ethlambda's QUIC drops while proving on the shared host) is a test-environment property and is not addressed here.

Validation

• cargo fmt --all --check, cargo clippy --locked --workspace --all-targets -- -D warnings, cargo test --locked --workspace, cargo deny --locked check — all green.

Closes #43. Closes #45.

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