[Perf][grounded-shop-assistant]: session eviction sorts all 5000 sessions on every new session once full #494

Affected Component

statewave-grounded-shop-assistant — server/src/sessionStore.ts

Description

Once the store is full, every new session copies and sorts the entire map just to find one entry to evict:

// server/src/sessionStore.ts:32-35
function evictOldestIfFull(): void {
  if (sessions.size < MAX_SESSIONS) return;
  const oldest = [...sessions.values()].sort((a, b) => a.lastSeen - b.lastSeen)[0];
  if (oldest) sessions.delete(oldest.id);
}

At MAX_SESSIONS = 5000, that's an array allocation plus an O(n log n) sort on the request path of every first message, which is exactly when the server is busiest (the map is full).

A JS Map iterates in insertion order. If a session is moved to the end whenever it's touched (delete then set), the first key is always the least-recently-used one, and eviction becomes O(1):

// on get(existing) and save():
sessions.delete(session.id);
sessions.set(session.id, session);

// evict:
const oldestId = sessions.keys().next().value;
if (oldestId !== undefined) sessions.delete(oldestId);

That also makes lastSeen only needed for the TTL sweep.

Expected Behavior

Evicting a session is O(1).

Suggested fix

As above. If the session-store tests from #493 land first, they'll confirm eviction order doesn't change.

Environment

main @ d4f0ac0.

Severity

Low — performance cleanup.