Adv 2.0 post-L5 audit cleanup: drop redundant dual-writes + refresh stale comments

Audit confirmed saveAdvCharacter's fan-out (upsertAllPlayerMetaFromAdvChar)
covers all 13 migrated subsystems, so the per-call-site upsertPlayerMetaXxx
calls retained "as defense" through the L4-L5 migration are redundant
post-L5h. Removed 16 such call sites across arena (2), scheduler (4),
hospital (1), consumables (2), rival (1), housing (11), and reordered
masterwork to drop a now-pointless explicit upsert before the load+save.

Refreshed nine stale doc comments referencing the legacy
adventure_characters table or "soak window / fallback" language that no
longer applies after the L5 close-out (commit 596b2b7). No behavior
change; go vet + go test ./... pass.

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

internal/plugin/adventure_rival.go

@@ -89,16 +89,12 @@ func (p *AdventurePlugin) checkRivalPoolUnlock(char *AdventureCharacter) {
 		return
 	}
 	// Promote to pool + send unlock DM if not yet announced.
+	// Persistence rides the caller's saveAdvCharacter → fan-out.
 	char.RivalPool = 1
-	pool = 1
 	if !notified {
 		char.RivalUnlockedNotified = true
-		notified = true
 		p.SendDM(char.UserID, rivalUnlockDM)
 	}
-	if err := upsertPlayerMetaRivalState(char.UserID, pool, notified); err != nil {
-		slog.Error("player_meta: rival unlock dual-write failed", "user", char.UserID, "err", err)
-	}
 }
 
 // ── DB CRUD ──────────────────────────────────────────────────────────────────