fix(ui): clear color registry on conversation end, add speaker color tests (#573)

- Reset _npc_entity_colors/_npc_entity_id/_next_npc_color in _end_player_conversation()
  to prevent palette exhaustion across long sessions with many unique NPCs
- Re-enforce contrast floor after passive desaturation (_enforce_contrast after _desaturate)
- Add TestDialogueSpeakerColors suite: palette allocation, entity reuse, reset, fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/ui/dialogue_box.gd

@@ -48,6 +48,8 @@ var _npc_name: String = ""
 # -- Entity color registry (#573) --
 # Maps entity_id → Color for dialogue participants.
 # Assigned from _npc_colors palette on first encounter; player uses _player_color.
+# v0.1: colors are per-conversation — cleared in _end_player_conversation() to avoid
+# palette exhaustion (8 entries) across long sessions with 9+ NPCs.
 var _npc_entity_colors: Dictionary = {}  # entity_id -> Color
 var _npc_entity_id: int = -1             # Entity ID of the current player conversation NPC
 var _next_npc_color: int = 0            # Round-robin palette index for client-side assignment
@@ -194,6 +196,7 @@ func _update_layout() -> void:
 ## is_passive: true for overheard NPC-NPC (renders with ┃ prefix + desaturated).
 ## Active conversation entries are pinned (no timeout) while _in_player_conversation.
 ## speaker_entity_id/target_entity_id: optional entity IDs for stable color lookup (#573).
+## TODO Phase 2: 6 positional params is unwieldy — consider dictionary-options overload.
 func append_line(speaker: String, target: String, text: String, is_passive: bool = false, speaker_entity_id: int = -1, target_entity_id: int = -1) -> void:
 	var pinned := not is_passive and _in_player_conversation
 	var entry: Dictionary = {
@@ -280,6 +283,8 @@ func append_player_line(target_npc: String, text: String) -> void:
 
 
 ## Append an NPC follow-up line (from dialogue_response).
+## Note: expects show_dialogue() to have been called first to set _npc_entity_id.
+## Defensive: if entity_id is valid but not yet registered, _assign_npc_color handles it.
 func append_dialogue_response(npc_name: String, text: String, entity_id: int = -1) -> void:
 	if entity_id >= 0:
 		_assign_npc_color(entity_id)
@@ -335,6 +340,11 @@ func _end_player_conversation() -> void:
 			entry.timestamp_msec = now
 	_log_dirty = true
 
+	# #573: Clear per-conversation color registry to avoid palette exhaustion
+	_npc_entity_colors.clear()
+	_npc_entity_id = -1
+	_next_npc_color = 0
+
 	# D-069: Clear dialogue/confrontation dip — coordinator routes to AudioManager
 	audio_dip_cleared.emit()
 
@@ -544,10 +554,10 @@ func _format_entry(entry: Dictionary, alpha: float) -> String:
 	var text: String = _escape_bbcode(entry.text)
 	var is_passive: bool = entry.is_passive
 
-	# Desaturate passive name colours (Araminta review)
+	# Desaturate passive name colours (Araminta review), re-enforce contrast floor after
 	if is_passive:
-		speaker_color = _desaturate(speaker_color, PASSIVE_DESATURATION)
-		target_color = _desaturate(target_color, PASSIVE_DESATURATION)
+		speaker_color = _enforce_contrast(_desaturate(speaker_color, PASSIVE_DESATURATION))
+		target_color = _enforce_contrast(_desaturate(target_color, PASSIVE_DESATURATION))
 
 	var sc := _color_with_alpha(speaker_color, alpha)
 	var ac := _color_with_alpha(_arrow_color, alpha)
@@ -576,6 +586,8 @@ static func _escape_bbcode(text: String) -> String:
 
 ## Assign a palette color to an NPC entity ID on first encounter (#573).
 ## Returns the same color on subsequent calls for the same entity ID.
+## TODO D-033 Phase 2: derive from relationship color — current independent palette
+## will need alignment when relationship-based entity colors arrive.
 func _assign_npc_color(entity_id: int) -> Color:
 	if entity_id < 0:
 		return _speech_color