fix(frontend): scope Call-to-Play event keys by author

Security audit finding Codex #14 ("cross-author event-ID collisions can
suppress Call-to-Play entries"). The Rust side guarantees that an event
nonce is unique within one authenticated author's history and preserves
`author_id` on every projected event, but the frontend reducer
deduplicated the merged view with `new Map(events.map(e => [e.id, e]))`
and tracked chat messages by `event.id` alone. A peer could therefore
publish, say, a Respond event reusing the nonce of another user's
Create event and make that call vanish from every viewer, or shadow
other users' chat messages.

`eventKeyOf` now builds `author_id + NUL + id` and is used for view
deduplication, event ordering ties, message deduplication and the
message id that CtpChat uses as its React key. Nomination ids
(`call_id`) were already creator scoped and are unchanged.

Test plan: `just frontend-test`. The new test feeds a Create from
Alice and a Respond from Bob sharing one nonce and expects both to
apply, then two same-nonce messages from different authors and expects
two distinct messages.

Claude-Session: https://claude.ai/code/session_017C3Nbgwpdm3YNwZhhFLHwg

crates/lanspread-tauri-deno-ts/src/lib/callToPlay.ts

@@ -50,8 +50,17 @@ interface MutableNomination extends Nomination {
     messageIds: Set<string>;
 }
 
+/**
+ * Event nonces are only unique within one author's history: the backend
+ * validates them per authenticated author, so two authors may legitimately
+ * (or deliberately) publish events with the same nonce. Every place that
+ * deduplicates or keys events therefore scopes the nonce by author.
+ */
+export const eventKeyOf = (event: Pick<CallToPlayViewEvent, 'author_id' | 'id'>): string =>
+    `${event.author_id}\u0000${event.id}`;
+
 const compareEvents = (a: CallToPlayViewEvent, b: CallToPlayViewEvent): number =>
-    a.at - b.at || a.id.localeCompare(b.id);
+    a.at - b.at || eventKeyOf(a).localeCompare(eventKeyOf(b));
 
 type CreatePayload = Extract<CallToPlayAction, { Create: unknown }>['Create'];
 type RespondPayload = Extract<CallToPlayAction, { Respond: unknown }>['Respond'];
@@ -135,7 +144,7 @@ export const reduceCallToPlayEvents = (
 const groupEvents = (
     input: ReadonlyArray<CallToPlayViewEvent>,
 ): Map<string, CallToPlayViewEvent[]> => {
-    const unique = new Map(input.map(event => [event.id, event]));
+    const unique = new Map(input.map(event => [eventKeyOf(event), event]));
     const byCall = new Map<string, CallToPlayViewEvent[]>();
     for (const event of unique.values()) {
         const events = byCall.get(event.call_id) ?? [];
@@ -218,10 +227,11 @@ const applyEvent = (nomination: MutableNomination, event: CallToPlayViewEvent):
     }
 
     const message = messagePayload(action);
-    if (message && !nomination.messageIds.has(event.id)) {
-        nomination.messageIds.add(event.id);
+    const messageKey = eventKeyOf(event);
+    if (message && !nomination.messageIds.has(messageKey)) {
+        nomination.messageIds.add(messageKey);
         nomination.messages.push({
-            id: event.id,
+            id: messageKey,
             fromId: event.author_id,
             from: event.author_name,
             text: message.text,

crates/lanspread-tauri-deno-ts/tests/callToPlay.test.ts

@@ -54,6 +54,27 @@ const create = (
     },
 });
 
+Deno.test('events from different authors never collapse on a shared nonce', () => {
+    const respond = event('create', 'Bob', { Respond: { ready_at: null } }, NOW + 1);
+    const [nomination] = reduceCallToPlayEvents([create(), respond], NOW + 2);
+    assert(nomination, 'the creator event must survive a colliding nonce from another author');
+    assertEquals(nomination.creator, 'Alice', 'creator');
+    assertEquals(Object.keys(nomination.participants).length, 2, 'both participants applied');
+
+    const aliceMessage = event('m1', 'Alice', { SendMessage: { text: 'hi' } }, NOW + 3);
+    const bobMessage = event('m1', 'Bob', { SendMessage: { text: 'yo' } }, NOW + 4);
+    const [withMessages] = reduceCallToPlayEvents(
+        [create(), respond, aliceMessage, bobMessage],
+        NOW + 5,
+    );
+    assert(withMessages, 'call should exist');
+    assertEquals(withMessages.messages.length, 2, 'messages with a shared nonce are distinct');
+    assert(
+        withMessages.messages[0].id !== withMessages.messages[1].id,
+        'message keys are author scoped',
+    );
+});
+
 Deno.test('play-now call starts with its creator ready', () => {
     const [nomination] = reduceCallToPlayEvents([create()], NOW);
     assert(nomination, 'call should exist');