Phase D: presence intent comment + sendZoneCombatMessages contract

D1: single block comment above the presence.go best-effort QueryRow
cluster (lines 149-198). Zero-value fallbacks are correct for new users
with no activity rows yet.

D2: contract comment on sendZoneCombatMessages — returns a done channel,
callers chaining post-flush work MUST consume it; in-flow handlers with
nothing to chain MUST explicitly discard. Blocking on flush would stall
command handlers behind the 2-3s/message pacing, defeating streaming.
Two fire-and-forget sites (adventure_arena.go round-advance,
dnd_zone_cmd.go streamFlow) now use `_ =` + comment.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/adventure_arena.go

@@ -429,7 +429,10 @@ func (p *AdventurePlugin) resolveArenaSurvival(ctx MessageContext, run *ArenaRun
 		finalMessage += "`!arena fight` — Face this opponent"
 	}
 
-	p.sendZoneCombatMessages(ctx.Sender, phaseMessages, finalMessage)
+	// Fire-and-forget: no post-flush work; blocking would stall the
+	// round-advance handler behind streamed pacing. Contract is honored
+	// by the explicit discard — see sendZoneCombatMessages comment.
+	_ = p.sendZoneCombatMessages(ctx.Sender, phaseMessages, finalMessage)
 	return nil
 }