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/data/dialogue-theme.yaml

@@ -0,0 +1,47 @@
+# Dialogue Log Theme — The Settled Reach
+#
+# Ticket: #535 | Sprint: 14
+# Colors and timing for the unified dialogue log panel.
+# Loaded by dialogue_box.gd at runtime.
+
+# ============================================================
+# PLAYER COLOR
+# Fixed color for the player's name in dialogue log entries.
+# ============================================================
+player_color: "#e0e8ff"
+
+# ============================================================
+# NPC COLOR PALETTE
+# 8 distinct colors for NPC names. Indexed by hash(npc_name) % 8.
+# Must be readable on a dark semi-transparent panel background.
+# ============================================================
+npc_colors:
+  0: "#4a9ebb"  # teal
+  1: "#6bc9a6"  # green
+  2: "#e8c547"  # amber
+  3: "#d49e5d"  # warm orange
+  4: "#b586d4"  # lavender
+  5: "#d45d5d"  # muted red
+  6: "#5daa7d"  # forest
+  7: "#7daccc"  # sky blue
+
+# ============================================================
+# TEXT COLORS
+# Arrow separator and speech text color.
+# ============================================================
+arrow_color: "#8890a0"
+speech_color: "#c8d0e0"
+
+# ============================================================
+# PASSIVE (OVERHEARD) OPACITY
+# Base opacity multiplier for non-player-centric lines.
+# 1.0 = full opacity, 0.0 = invisible.
+# ============================================================
+passive_opacity: "0.9"
+
+# ============================================================
+# ENTRY TIMING
+# All entry types share the same lifetime and fade duration.
+# ============================================================
+entry_lifetime_seconds: "15.0"
+entry_fade_seconds: "3.0"