[state] Add EventHash → index map for O(1) message lookup

[intendednull]

Parent: #108

Problem

crates/state/src/materialize.rs does a linear scan over state.messages every time a message-targeting event is applied:

• Line 369 (EditMessage): state.messages.iter_mut().find(|m| m.id == *message_id)
• Line 376 (DeleteMessage): same pattern
• Line 384 (Reaction): same pattern

For a channel with N messages, every edit/delete/reaction is O(N). At 10k messages and a reaction flood, this dominates replay time.

Fix

Add a BTreeMap<EventHash, usize> field on ServerState mapping message hash → index into state.messages. Update it on every Message insertion and use it for lookup in the three sites above.

pub struct ServerState {
    pub messages: Vec<ChatMessage>,
    pub message_index: BTreeMap<EventHash, usize>,
    // ...
}

Watch out for:

• Ensuring the index is rebuilt correctly on materialize() replay.
• DeleteMessage currently keeps the message in messages (sets deleted = true), so indexes stay stable. Good.
• If any future code removes entries from messages, the index needs maintenance.

Test

#[test]
fn message_lookup_is_constant_time() {
    // Build a state with 100k messages, then time a reaction apply.
    // Prior: ~100ms per reaction. After: should be <100µs.
}

Also add a determinism test: materialize a DAG twice and assert the message_index maps match.

[intendednull]

Already implemented in commit ab78113 (merged via #146): added message_index: HashMap<EventHash, usize> on ServerState for O(1) lookup in EditMessage / DeleteMessage / Reaction handlers, with the index rebuilt deterministically in materialize(). Closing as resolved.