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/protocol/protocol.gd

@@ -1,4 +1,4 @@
-class_name Protocol
+extends Node
 ## MessagePack codec for the Rust↔Godot wire protocol (D-020).
 ##
 ## Encodes/decodes ObserverSnapshot and PlayerInput to match
@@ -11,8 +11,8 @@ class_name Protocol
 
 ## Protocol version — must match server PROTOCOL_VERSION in bridge/types.rs.
 ## Reject snapshots where version != this value.
-## v19: adds character_archetype field to StartupMessage (#588, #587).
-const PROTOCOL_VERSION: int = 19
+## v20: adds settings_response field to ObserverSnapshot (#627, D-138).
+const PROTOCOL_VERSION: int = 20
 
 
 # -- Decode: bytes from server → GDScript types --------------------------------
@@ -270,6 +270,32 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 					"triangle_id": int(raw_ev["triangle_id"]),
 				})
 
+	# v20: settings_response (#627, D-138) — server ack after ChangeSettings / full settings dump
+	# after RequestAllSettings. kind = "full"|"ack". "full": settings array [{key, value}].
+	# "ack": {success: bool, key: String, error: String|null}.
+	var settings_response: Variant = null
+	var raw_sr: Variant = raw.get("settings_response")
+	if raw_sr is Dictionary:
+		var sr_kind: String = str(raw_sr.get("kind", ""))
+		if sr_kind == "full":
+			var sr_settings: Array = []
+			var raw_sr_settings: Variant = raw_sr.get("settings")
+			if raw_sr_settings is Array:
+				for raw_s in raw_sr_settings:
+					if raw_s is Dictionary and raw_s.has("key"):
+						sr_settings.append({
+							"key": str(raw_s["key"]),
+							"value": raw_s.get("value"),
+						})
+			settings_response = {"kind": "full", "settings": sr_settings}
+		elif sr_kind == "ack":
+			settings_response = {
+				"kind": "ack",
+				"success": bool(raw_sr.get("success", false)),
+				"key": str(raw_sr.get("key", "")),
+				"error": raw_sr.get("error"),
+			}
+
 	# v19: current_ticker (#592) — scrolling news headline when in The Last Shift zone.
 	# {id: String, text: String, category: String} or null when player outside bar zone.
 	var current_ticker: Variant = null
@@ -362,6 +388,7 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		"zone_id": zone_id,
 		"triangle_crisis_events": triangle_crisis_events,
 		"current_ticker": current_ticker,
+		"settings_response": settings_response,
 	}
 
 
@@ -548,6 +575,24 @@ static func _encode_action(action_name: String, action_data: Variant) -> Variant
 	return action_name
 
 
+## Encode a ChangeSettings action for the AI-Enhanced Dialogue toggle (#646, D-138).
+## Returns a MessagePack-encoded Vec<PlayerInput> in buffer-entry format
+## (action_name + action_data), suitable for inspection in tests and for
+## queuing via SimBridge._outbound_buffer.
+## The server receives this as a PlayerAction::ChangeSettings variant.
+static func encode_change_settings(enabled: bool) -> PackedByteArray:
+	var entries: Array = [{
+		"tick": 0,
+		"action_name": "ChangeSettings",
+		"action_data": {"ai_enhanced_dialogue": enabled},
+	}]
+	var result = Messagepack.encode(entries)
+	if result.status != null:
+		push_error("Protocol: encode_change_settings failed: %s" % result.status)
+		return PackedByteArray()
+	return result.value
+
+
 ## Decode a PlayerInput from MessagePack bytes (used in tests / echo scenarios).
 ## Returns { "tick": int, "action": { "variant": String, "data": Variant } } or null.
 static func decode_player_input(bytes: PackedByteArray) -> Variant: