feat(ui): implement dialogue pipeline — selection, walk-away, confrontation

Protocol: decode current_dialogue with structured options {text,
response_id, priority, confrontation} and npc_entity_id (#435).
Dialogue box: priority sort, max 3 visible, RichTextLabel for BBCode
italic confrontation options (D-063), 1.5s monologue beat with audio
dip before confrontation send. Walk-away: WASD triggers WalkAway
input, 300ms fade, dialogue_active flag gates movement (D-064).
Implements #435, #437, #436.

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

client/scripts/autoloads/input_mapper.gd

@@ -31,7 +31,10 @@ var _last_move_msec: int = 0
 
 # Hold-to-move: poll held direction keys each frame, throttled by stance.
 # Server-side cooldown (D-053) is authoritative; this prevents client flooding.
+# D-064: movement suppressed during dialogue (walk-away handled by dialogue_box).
 func _process(_delta: float) -> void:
+	if GameState.dialogue_active:
+		return
 	var dir := Vector2i.ZERO
 	if Input.is_action_pressed("move_north"):
 		dir.y -= 1