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/tests/test_dialogue_speaker_colors.gd

@@ -0,0 +1,153 @@
+## Sprint 23 #573: dialogue speaker color binding tests.
+##
+## Verifies entity-ID-bound speaker color assignment in dialogue_box.gd:
+## round-robin palette allocation, same-entity reuse, conversation-end reset,
+## and fallback behavior when no entity ID is provided.
+##
+## D-030: fixture-based, server-free, no subprocess required.
+class_name TestDialogueSpeakerColors
+extends GdUnitTestSuite
+
+
+func _make_dialogue_box() -> Control:
+	if not ResourceLoader.exists("res://ui/dialogue_box.tscn"):
+		push_warning("TestDialogueSpeakerColors: dialogue_box.tscn not found — scene tests skipped")
+		return null
+	var node: Control = load("res://ui/dialogue_box.tscn").instantiate()
+	add_child(node)
+	return node
+
+
+func before_test() -> void:
+	GameState.dialogue_active = false
+
+
+func after_test() -> void:
+	GameState.dialogue_active = false
+
+
+# -- _assign_npc_color: round-robin assignment ---------------------------------
+
+func test_assign_npc_color_returns_palette_color() -> void:
+	## First call for an entity ID should return a color from the NPC palette.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var color: Color = box._assign_npc_color(100)
+	assert_that(color).override_failure_message(
+		"_assign_npc_color must return a non-default color for a valid entity ID (#573)"
+	).is_not_equal(box._speech_color)
+
+
+func test_assign_npc_color_same_entity_returns_same_color() -> void:
+	## Repeated calls for the same entity ID must return the same color.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var color1: Color = box._assign_npc_color(200)
+	var color2: Color = box._assign_npc_color(200)
+	assert_that(color1).override_failure_message(
+		"_assign_npc_color must return the same color for the same entity ID (#573)"
+	).is_equal(color2)
+
+
+func test_assign_npc_color_different_entities_get_different_colors() -> void:
+	## Different entity IDs should get different colors (within palette size).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var color1: Color = box._assign_npc_color(300)
+	var color2: Color = box._assign_npc_color(301)
+	assert_that(color1).override_failure_message(
+		"Different entity IDs must get different palette colors (#573)"
+	).is_not_equal(color2)
+
+
+func test_assign_npc_color_negative_id_returns_speech_color() -> void:
+	## Negative entity ID (no entity) should fall back to _speech_color.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var color: Color = box._assign_npc_color(-1)
+	assert_that(color).override_failure_message(
+		"_assign_npc_color(-1) must return _speech_color fallback (#573)"
+	).is_equal(box._speech_color)
+
+
+# -- Color registry cleared on conversation end --------------------------------
+
+func test_color_registry_cleared_on_conversation_end() -> void:
+	## After hide_dialogue(), the color registry must be empty so next
+	## conversation starts fresh (avoids palette exhaustion).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.show_dialogue("NPC", "Hello.", [], 400)
+	assert_bool(box._npc_entity_colors.has(400)).override_failure_message(
+		"Entity color should be registered during conversation (#573)"
+	).is_true()
+	box.hide_dialogue()
+	assert_bool(box._npc_entity_colors.is_empty()).override_failure_message(
+		"_npc_entity_colors must be cleared after conversation ends (#573)"
+	).is_true()
+	assert_int(box._next_npc_color).override_failure_message(
+		"_next_npc_color must reset to 0 after conversation ends (#573)"
+	).is_equal(0)
+
+
+func test_color_registry_reset_gives_fresh_assignment() -> void:
+	## After conversation end + new conversation, same entity ID gets a color
+	## (may differ from previous conversation — that's fine, per-conversation).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.show_dialogue("NPC", "Hello.", [], 500)
+	var color1: Color = box._npc_entity_colors.get(500, Color.BLACK)
+	box.hide_dialogue()
+	box.show_dialogue("NPC", "Hi again.", [], 500)
+	var color2: Color = box._npc_entity_colors.get(500, Color.BLACK)
+	# Both should be valid palette colors (not BLACK fallback)
+	assert_that(color1).override_failure_message(
+		"First conversation color must be a palette color (#573)"
+	).is_not_equal(Color.BLACK)
+	assert_that(color2).override_failure_message(
+		"Second conversation color must be a palette color (#573)"
+	).is_not_equal(Color.BLACK)
+
+
+# -- show_dialogue entity ID threading ----------------------------------------
+
+func test_show_dialogue_registers_npc_color() -> void:
+	## show_dialogue with a valid entity ID must register the color.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.show_dialogue("Kael", "Welcome.", [], 600)
+	assert_bool(box._npc_entity_colors.has(600)).override_failure_message(
+		"show_dialogue must register entity color when npc_entity_id provided (#573)"
+	).is_true()
+
+
+func test_show_dialogue_without_entity_id_no_registration() -> void:
+	## show_dialogue without entity ID should not register any color.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.show_dialogue("NPC", "Hello.", [])
+	assert_bool(box._npc_entity_colors.is_empty()).override_failure_message(
+		"show_dialogue without entity ID must not register colors (#573)"
+	).is_true()
+
+
+# -- append_dialogue_response defensive guard ----------------------------------
+
+func test_append_dialogue_response_registers_color_if_missing() -> void:
+	## append_dialogue_response with a valid entity_id must register the color
+	## even if show_dialogue was not called first (defensive guard).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.append_dialogue_response("Voss", "I see.", 700)
+	assert_bool(box._npc_entity_colors.has(700)).override_failure_message(
+		"append_dialogue_response must register color for unknown entity ID (#573)"
+	).is_true()