feat(client): save/load client UI — F5/F6 quicksave/quickload (#554)

Wire SaveGame/LoadGame player actions through the full client stack:
protocol v15 decode, InputMapper F5/F6 bindings, SimBridge wire mapping
with one-shot carry-forward, GameState save_result field, and HUD
notification via monologue display. Quit-to-menu triggers quicksave
before scene change.

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

client/ui/monologue_display.gd

@@ -35,6 +35,8 @@ const _LATTICE_COLORS: Dictionary = {
 }
 const _FALLBACK_STANDARD: Color = Color("#c8d0e0")
 const _FALLBACK_URGENT:   Color = Color("#e0e8f8")
+const _NOTIFICATION_COLOR: Color = Color("#8890a0")  # #554: neutral system notification
+const _NOTIFICATION_DURATION: float = 2.5
 
 @onready var _vbox: VBoxContainer = $VBoxContainer
 
@@ -71,6 +73,18 @@ func _process(delta: float) -> void:
 # Empty text is silently ignored — no slot created, no queue entry.
 # lattice_profile is read from GameState here and passed down — renderer stays
 # decoupled from the autoload (D-020 renderer contract).
+# #554: Show a brief system notification (save/load result, connection status).
+# Uses neutral color, short duration, bypasses lattice_profile styling.
+func show_notification(text: String) -> void:
+	if text.is_empty():
+		return
+	var now := float(Time.get_ticks_msec())
+	if _visible.size() < MAX_VISIBLE and now >= _next_fade_in_msec:
+		_show_notification_line(text)
+	else:
+		_enqueue(text, _NOTIFICATION_DURATION, 1, false, "")
+
+
 func show_monologue(text: String, duration: float, priority: int = 2, is_urgent: bool = false) -> void:
 	if text.is_empty():
 		return
@@ -86,6 +100,35 @@ func show_monologue(text: String, duration: float, priority: int = 2, is_urgent:
 # Internal
 # ---------------------------------------------------------------------------
 
+func _show_notification_line(text: String) -> void:
+	var container := MarginContainer.new()
+	container.add_theme_constant_override("margin_left",   4)
+	container.add_theme_constant_override("margin_right",  4)
+	container.add_theme_constant_override("margin_top",    2)
+	container.add_theme_constant_override("margin_bottom", 2)
+	var label := RichTextLabel.new()
+	label.bbcode_enabled = true
+	label.fit_content = true
+	label.scroll_active = false
+	label.add_theme_font_size_override("normal_font_size", 13)
+	var safe_text := text.replace("[", "[lb]")
+	label.text = "[color=#%s]%s[/color]" % [_NOTIFICATION_COLOR.to_html(false), safe_text]
+	container.add_child(label)
+	_vbox.add_child(container)
+	var slot := {
+		node         = container,
+		expire_timer = _NOTIFICATION_DURATION,
+		priority     = 1,
+		tween        = null,
+	}
+	_visible.append(slot)
+	_next_fade_in_msec = float(Time.get_ticks_msec()) + STAGGER_SEC * 1000.0
+	container.modulate.a = 0.0
+	var tween := create_tween()
+	slot.tween = tween
+	tween.tween_property(container, "modulate:a", 0.85, FADE_IN_SEC)
+
+
 func _show_line(text: String, duration: float, priority: int, is_urgent: bool, lattice_profile: String) -> void:
 	var line_node := _build_line_node(text, is_urgent, lattice_profile)
 	_vbox.add_child(line_node)