Adv 2.0 L4f-prep: flip DisplayName readers to loadDisplayName

Swap all char.DisplayName / c.DisplayName reader sites in
internal/plugin/ to loadDisplayName(userID). Touches 12 files
across combat (combat_bridge, dnd_zone_combat, dnd_zone_cmd,
dnd_expedition_combat), arena, hospital, events, render,
masterwork, robbie, scheduler, and adventure.go.

Only intentional char.DisplayName references remaining are in
adventure_character.go (scan + save + dual-write) and the
player_meta.go doc comment. go vet + go test ./internal/plugin/...
clean.

Unblocks deferred L2 step 9 (arena AdvCharacter import drop)
once L4 hospital/pets/render also lands.

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

internal/plugin/dnd_zone_combat.go

@@ -164,8 +164,9 @@ func (p *AdventurePlugin) runZoneCombat(
 	}
 	applyPendingCast(userID, dndChar, &playerStats, &playerMods, &enemyStats)
 
+	displayName, _ := loadDisplayName(userID)
 	player := Combatant{
-		Name:     char.DisplayName,
+		Name:     displayName,
 		Stats:    playerStats,
 		Mods:     playerMods,
 		IsPlayer: true,