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

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

client/tests/unit/test_fallback_library.gd

@@ -0,0 +1,33 @@
+extends "res://addons/gut/test.gd"
+
+const FallbackLibrary = preload("res://scripts/net/fallback_library.gd")
+
+
+func test_loads_authored_line_from_default_file():
+	var lib = FallbackLibrary.new()
+	# The authored line is distinct from LAST_RESORT, so a match proves the file was read.
+	assert_ne(lib.narrator_line(), FallbackLibrary.LAST_RESORT)
+	assert_true(lib.narrator_line().length() > 0)
+
+
+func test_missing_file_uses_last_resort():
+	var lib = FallbackLibrary.new("res://content/fallback/does_not_exist.json")
+	assert_eq(lib.narrator_line(), FallbackLibrary.LAST_RESORT)
+
+
+func test_malformed_file_uses_last_resort():
+	var path := "user://bad_fallback.json"
+	var f := FileAccess.open(path, FileAccess.WRITE)
+	f.store_string("{ this is not valid json")
+	f.close()
+	var lib = FallbackLibrary.new(path)
+	assert_eq(lib.narrator_line(), FallbackLibrary.LAST_RESORT)
+
+
+func test_empty_list_uses_last_resort():
+	var path := "user://empty_fallback.json"
+	var f := FileAccess.open(path, FileAccess.WRITE)
+	f.store_string('{"narrator": []}')
+	f.close()
+	var lib = FallbackLibrary.new(path)
+	assert_eq(lib.narrator_line(), FallbackLibrary.LAST_RESORT)