Function select_protocol 

pub fn select_protocol(
    local: &SyncHandshake,
    remote: &SyncHandshake,
) -> ProtocolSelection

Select the optimal sync protocol based on handshake information.

Implements the decision table from CIP §2.3:

#	Condition	Selected Protocol
1	root_hash match	None
2	!has_state (fresh node)	Snapshot
3	has_state AND divergence >50%	HashComparison
4	max_depth >3 AND divergence <20%	SubtreePrefetch
5	entity_count >50 AND divergence <10%	BloomFilter
6	max_depth 1-2 AND avg children/level >10	LevelWise
7	(default)	HashComparison

CRITICAL (Invariant I5): Snapshot is NEVER selected for initialized nodes. This prevents silent data loss from overwriting local CRDT state.