fix(orbit): host-side session guards — settings default, maxUsers, push serialisation (#1157)

* fix(orbit): unify session-settings default to the canonical preset

updateOrbitSettings merged patches onto a hand-rolled
{ autoApprove: true, autoShuffle: true } fallback when a legacy session's
blob predated the settings field. That literal disagreed with
ORBIT_DEFAULT_SETTINGS (autoApprove: false), so toggling autoShuffle on
such a session silently flipped autoApprove on, landing guest suggestions
without host approval. Use ORBIT_DEFAULT_SETTINGS as the single source of
truth for the fallback. Add a behavioural test for the settings-less,
preserve-existing, and not-hosting paths.

* fix(orbit): enforce maxUsers host-side in the participant rebuild

The join gate only sees a one-tick-old state blob, so two guests can both
pass the participants.length < maxUsers check and join past the cap at
once; maxUsers was advisory. The host is the single writer of the
canonical state, so enforce the cap there: admit at most maxUsers guests
in the participant rebuild, earliest joiners winning (an established
participant is never displaced by a newcomer) with a deterministic
username tie-break for same-tick joins. Suggestions from an over-cap guest
are ignored, consistent with their absence from the participant list.
Reorders the fold so the admitted set gates the queue-additions loop. Add
tests for the cap, the no-displacement rule, and the ignored over-cap
suggestion.

* fix(orbit): serialise host state pushes to stop overlapping ticks dropping writes

pushState does several awaited round-trips (sweep -> resolve -> write) and
is fired from three sources: the mount, the 2.5 s timer, and a play/pause
subscription. With no in-flight lock these can overlap and race
last-writer-wins: a slow run that already swept and cleared an outbox can
lose its write to a faster run, dropping those suggestions from both the
outbox and the published state. The lastPushedAtRef was only ever written,
never used as a guard.

Add makeCoalescedRunner: a pure helper that serialises an async task and
coalesces any mid-flight request into a single rerun, so no trigger is
lost. The host hook wraps pushState in it; the dead ref is removed. Add a
unit test for the serialise + coalesce semantics.

* docs(changelog): add Orbit host-side session guards (1157)

src/utils/orbit.ts

@@ -27,6 +27,7 @@ export {
 } from './orbit/constants';
 export {
   generateSessionId,
+  makeCoalescedRunner,
   OrbitStateTooLarge,
   serialiseOrbitState,
   suggestionKey,