feat(ui): unified dialogue log with overheard NPC conversations (#535)

Refactors dialogue box into a scrolling conversation log. All dialogue
(player-NPC and overheard NPC-NPC) flows chronologically, oldest at top.
Player response options at the bottom during active conversations.

- Entries expire after configurable timeout (equal for all message types)
- Walk-away clears options but preserves log entries (fair information)
- Per-character name colors from dialogue-theme.yaml (hash-indexed palette)
- Overheard lines render at 90% opacity (D-078)
- Protocol decode for conversation_events + conversation_ended
- GameState fields for conversation_events, conversation_ended, dialogue_response
- Mock Mira/Soren NPC-NPC conversation in test snapshot
- Also wires #511 debug overlay into main.gd and main.tscn

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

client/scenes/main.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=22 format=3 uid="uid://bswrmh7w8dbgm"]
+[gd_scene load_steps=23 format=3 uid="uid://bswrmh7w8dbgm"]
 
 [ext_resource type="Script" path="res://scripts/main.gd" id="1_main"]
 [ext_resource type="Script" path="res://scripts/rendering/world_renderer.gd" id="2_world"]
@@ -21,6 +21,7 @@
 [ext_resource type="PackedScene" path="res://ui/checklist_overlay.tscn" id="18_checklist"]
 [ext_resource type="PackedScene" path="res://ui/bug_report_dialog.tscn" id="19_bugreport"]
 [ext_resource type="PackedScene" path="res://ui/settings_dialog.tscn" id="21_settings"]
+[ext_resource type="Script" path="res://scripts/ui/debug_overlay.gd" id="22_debug"]
 
 [node name="Game" type="Node2D"]
 script = ExtResource("1_main")
@@ -152,6 +153,16 @@ layer = 20
 ; D-065: Inventory grid — 3x3, bottom-right, 40x40px, 1-9 hotkeys
 [node name="InventoryGrid" parent="UILayer" instance=ExtResource("12_inv")]
 
+; #511: F3 debug overlay — real-time game state, toggled by F3
+[node name="DebugOverlay" type="Control" parent="UILayer"]
+anchors_preset = 0
+offset_left = 16
+offset_top = 120
+offset_right = 400
+offset_bottom = 400
+mouse_filter = 2
+script = ExtResource("22_debug")
+
 ; D-056: Cursor state machine — insert-styled geometric cursor, topmost in UILayer
 [node name="CursorRenderer" type="Node2D" parent="UILayer"]
 script = ExtResource("10_cursor")