fix: reduce hot-path allocation, cloning, and lock contention

[varex83agent]

Summary

Implements the eight hot-path optimizations from the hot-path allocation, cloning, and lock-contention reduction plan. Every change is perf-only and preserves functional equivalence with Charon v1.7.1 — no wire format, validation, ordering, or metric semantics change.

Tasks

Task	Crate	Change
T01	p2p	Add alloc-free PeerStore::has_connection; migrate the 5 emptiness-only connections_to_peer(..).is_empty() callers (qbft, frostp2p, bcast, parsigex, monitoringapi).
T02	consensus (qbft)	Thread one Arc<QbftConsensusMsg> across all peers on broadcast instead of an N-1 deep clone of the multi-MB payload; move msg/justification/values out of pb_msg on receive; values_by_hash now consumes Vec<Any>.
T03	eth2api	Arc-wrap the cached ActiveValidators/CompleteValidators maps (cheap clones via Deref), add a read-lock fast path on a warm cache, and release the write lock across the beacon-node fetch (re-checking on re-acquire).
T04	core (scheduler)	Replace the O(N) to_entries() status-metric scan with a per-pubkey last-status map, zeroing only the previously-reported series — mirrors Charon's statusGauge.Reset.
T05	core (scheduler)	Cache slots_per_epoch on the actor at build time instead of a beacon-node round-trip on every duty lookup.
T06	parsigex	Acquire the peer-store guard once per broadcast (using T01's has_connection) and share the encoded payload via Arc<[u8]> instead of a per-target Vec<u8> copy.
T07	core (parsigdb)	Fold the threshold check into store under the lock and return only the matched set via a StoreOutcome enum, dropping the per-store full-accumulator deep clone.
T08	ssz	Pass the buffer by value into merkleize_impl (one copy instead of two); reuse a single Sha256 in default_hash_fn; drop the put_bitlist scratch clone.

Tests

New unit/regression tests added for each task (peer-store connectivity, Arc::ptr_eq shared-buffer assertions for qbft and parsigex, concurrent valcache misses, per-pubkey status-metric transitions, cached-slots_per_epoch epoch math, parsigdb duplicate/mismatch paths, and ssz golden merkleization roots).

Quality gates

• cargo +nightly fmt --all --check ✅
• cargo clippy --workspace --all-targets --all-features -- -D warnings ✅
• cargo test --workspace --all-features ✅ (all suites pass)
• cargo deny check ✅

🤖 Generated with Claude Code