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/scripts/autoloads/game_state.gd

@@ -70,6 +70,11 @@ var insert_active: bool = true
 # Null in v0.1 (server does not yet send this field; protocol change required).
 var rng_seed: Variant = null
 
+# v15 fields (#554, D-085): save/load result from server.
+# {success: bool, kind: "save"|"load", error: Variant} or null.
+# One-shot: consumed by main.gd after display, then set back to null.
+var save_result: Variant = null
+
 # v7 fields (#431, D-059/D-060)
 var pending_recognitions: Array = []  # [{entity_id, x, y, z, remaining_ticks, total_delay_ticks}]
 
@@ -279,6 +284,12 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	else:
 		current_examine_result = null
 
+	# v15: save_result (#554, D-085) — one-shot save/load confirmation from server.
+	if snapshot.has("save_result") and snapshot.save_result is Dictionary:
+		save_result = snapshot.save_result
+	else:
+		save_result = 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: