state: dedupe Reaction events by author

[intendednull]

Summary

• Switch ChatMessage::reactions from BTreeMap<String, Vec<EndpointId>> to BTreeMap<String, BTreeSet<EndpointId>> so each peer can only be registered once per (message, emoji).
• Change the EventKind::Reaction handler in materialize.rs to use BTreeSet::insert instead of Vec::push, eliminating duplicate reactor entries when the same author emits (or replays) the same Reaction event twice.
• Add a state-machine regression test same_author_duplicate_reaction_is_idempotent and tighten the existing duplicate_reaction_from_same_peer assertion from "may dedupe or not" to "must dedupe exactly".

Why

The previous Vec-backed shape meant a retransmitted or replayed Reaction would inflate the reactor list, causing UI reaction miscounts and StateHash divergence between peers that received different delivery counts of the same event. Deduping by author in the materialized state is both the simplest fix and the most compatible — the wire format of EventKind::Reaction is unchanged.

Scope notes

• The wire/event format is untouched. Only the in-memory materialized state shape changed.
• DisplayMessage.reactions in willow-client is a separate HashMap<String, Vec<String>> of display names and is unaffected.
• Consumers that iterate .reactions (e.g. willow-client::views::messages_view) work unchanged because they only use .iter().

Test plan

• cargo test -p willow-state — 156 tests pass including the new same_author_duplicate_reaction_is_idempotent regression
• cargo test -p willow-client — 66 tests pass
• cargo clippy --workspace -- -D warnings — clean
• cargo fmt --check — clean
• cargo check --target wasm32-unknown-unknown for the state/client/channel/messaging/crypto/identity/transport/common/network/actor crates — clean

Closes #111
Progresses #108