feat(client): FallbackLibrary + authored narrator fallback (§13)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scripts/net/fallback_library.gd

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+class_name FallbackLibrary
+extends RefCounted
+## Authored degraded-DM fallback text (charter §13). The client shows one of
+## these lines on ANY failure, in the Narrator's voice — never an error. If the
+## content file is missing or malformed the library returns a hardcoded
+## constant, so the fallback itself cannot fail.
+
+const PATH := "res://content/fallback/narrator.json"
+const LAST_RESORT := "The chamber is cold. Something waits in the dark."
+
+var _lines: Array = []
+
+
+func _init(path := PATH) -> void:
+	_lines = _load(path)
+
+
+func narrator_line() -> String:
+	return str(_lines[0]) if not _lines.is_empty() else LAST_RESORT
+
+
+static func _load(path: String) -> Array:
+	if not FileAccess.file_exists(path):
+		return []
+	var text := FileAccess.get_file_as_string(path)
+	# Use the instance API, not JSON.parse_string(): the static convenience
+	# method pushes an engine-level error on malformed input, which is noisy
+	# and (in this project's test config) turns a handled failure into a
+	# reported test error. The instance parse() just returns an Error code.
+	var json := JSON.new()
+	if json.parse(text) != OK:
+		return []
+	var data = json.get_data()
+	if typeof(data) != TYPE_DICTIONARY:
+		return []
+	var lines = data.get("narrator", [])
+	return lines if typeof(lines) == TYPE_ARRAY else []

client/scripts/net/fallback_library.gd.uid

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+uid://b8jhu0e4yg6tf