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/adventure_scheduler.go

@@ -232,8 +232,9 @@ func (p *AdventurePlugin) postDailySummary() {
 	// Build player summaries
 	var players []AdvPlayerDaySummary
 	for _, c := range chars {
+		dispName, _ := loadDisplayName(c.UserID)
 		ps := AdvPlayerDaySummary{
-			DisplayName:   c.DisplayName,
+			DisplayName:   dispName,
 			CombatLevel:   c.CombatLevel,
 			MiningSkill:   c.MiningSkill,
 			ForagingSkill: c.ForagingSkill,