feat(client): authored NPC fallback line (§13)

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

client/tests/unit/test_fallback_library.gd

@@ -31,3 +31,15 @@ func test_empty_list_uses_last_resort():
 	f.close()
 	var lib = FallbackLibrary.new(path)
 	assert_eq(lib.narrator_line(), FallbackLibrary.LAST_RESORT)
+
+
+func test_npc_line_falls_back_to_constant_when_missing():
+	var lib = FallbackLibrary.new(
+		"res://content/fallback/does_not_exist.json",
+		"res://content/fallback/does_not_exist.json")
+	assert_eq(lib.npc_line(), FallbackLibrary.NPC_LAST_RESORT)
+
+
+func test_npc_line_reads_authored_file():
+	var lib = FallbackLibrary.new()  # default paths — real content
+	assert_ne(lib.npc_line(), "")