fix(channels): pending message compaction never triggering

Move count assignment before RAM trim in Record() so MaybeCompact
receives pre-trim count (51) that exceeds threshold (50), allowing
compaction to fire. Previously count was always ≤ limit after trim.

internal/channels/history.go

@@ -120,11 +120,11 @@ func (ph *PendingHistory) Record(historyKey string, entry HistoryEntry, limit in
 	ph.mu.Lock()
 	existing := ph.entries[historyKey]
 	existing = append(existing, entry)
+	count = len(existing) // capture pre-trim count so MaybeCompact sees threshold exceeded
 	if len(existing) > limit {
 		existing = existing[len(existing)-limit:]
 	}
 	ph.entries[historyKey] = existing
-	count = len(existing)
 	ph.removeFromOrder(historyKey)
 	ph.order = append(ph.order, historyKey)
 	ph.evictOldKeys()