feat(ui): AI-Enhanced Dialogue toggle + hardware detection (#646, D-138)

Implements the full AI-Enhanced Dialogue feature for Sprint 26:

- HardwareDetector autoload (extends Node): three-layer detection —
  Layer 1 RAM classification (pass/marginal/fail, thresholds 2GB/1.6GB),
  Layer 2 TPT benchmark cache (green/yellow/red, thresholds 6/3 t/s),
  Layer 3 degradation monitoring (>40% drop → yellow). load_ai_pref()
  restores toggle state from user://settings.cfg on startup.

- Settings dialog AI section: toggle, colored status dot, status message
  label, testable API (get_ai_dialogue_label_text, set_ai_dialogue_
  hardware_status, is_ai_dialogue_toggle_enabled). Only RAM "fail" greys
  out toggle — player always overrides yellow/red recommendations (D-138).

- GameState.ai_enhanced_dialogue_enabled (default true, opt-out model).
  GameState.settings_response (v20 one-shot settings dump from server).
  apply_snapshot() hydrates ai_enhanced_dialogue_enabled from full dump.

- Protocol v19→v20, settings_response decoding in decode_snapshot().
  Protocol converted to extends Node autoload (enables test has_method).
  encode_change_settings() helper for test inspection.

- InputMapper: CHANGE_SETTINGS, REQUEST_ALL_SETTINGS, DELETE_SETTING.
  SimBridge: wire mappings for all three. RequestAllSettings queued after
  handshake to hydrate client state from server SQLite on connect.
  settings_response carry-forward in receive_bytes().

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

client/scripts/autoloads/game_state.gd

@@ -97,6 +97,17 @@ var pending_load_path: String = ""
 # Default: "detective" — fallback for legacy saves without character.txt.
 var character_archetype: String = "detective"
 
+# #646: AI-Enhanced Dialogue enabled state (D-138).
+# Runtime toggle — true means the LLM re-voicing pipeline should run (server-side).
+# Default: true (opt-out model per D-138 §8). Hardware detector may disable at startup
+# if RAM is insufficient. Persisted to server SQLite via ChangeSettings IPC.
+var ai_enhanced_dialogue_enabled: bool = true
+
+# v20 fields (#627, D-138): settings response from server.
+# One-shot: {kind: "full", settings: [{key, value}]} or {kind: "ack", success, key, error} or null.
+# "full" response hydrates ai_enhanced_dialogue_enabled (server is authoritative for persisted state).
+var settings_response: Variant = null
+
 # v7 fields (#431, D-059/D-060)
 var pending_recognitions: Array = []  # [{entity_id, x, y, z, remaining_ticks, total_delay_ticks}]
 
@@ -328,6 +339,24 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	else:
 		debug_response = null
 
+	# v20: settings_response (#627, D-138) — one-shot settings ack/dump from server.
+	# "full" kind → iterate settings array and hydrate matching fields.
+	if snapshot.has("settings_response") and snapshot.settings_response is Dictionary:
+		settings_response = snapshot.settings_response
+		var sr: Dictionary = snapshot.settings_response
+		if sr.get("kind") == "full":
+			var sr_settings: Variant = sr.get("settings")
+			if sr_settings is Array:
+				for entry in sr_settings:
+					if not entry is Dictionary:
+						continue
+					if entry.get("key") == "ai_dialogue.enabled":
+						var val: Variant = entry.get("value")
+						if val is Dictionary and val.has("Bool"):
+							ai_enhanced_dialogue_enabled = bool(val["Bool"])
+	else:
+		settings_response = null
+
 	# v14: player_knowledge (#264, D-041) — partial KG dump for journal panel.
 	# Only update when field is present (null means no change, server sends when KG changes).
 	if snapshot.has("player_knowledge") and snapshot.player_knowledge is Dictionary: