migration: prove standalone maintenance and correct visual evidence

tests/preservation_helpers.py

@@ -26,6 +26,7 @@ RETIRED_FRONTEND_SOURCE_RANGES = (
     (9022, 9027),
     (9052, 9055),
 )
+AUDITED_FRONTEND_SOURCE_LINE_COUNT = 9283
 
 
 def sha256(path: Path) -> str:
@@ -51,12 +52,10 @@ def reassembled_frontend_runtime() -> str:
         assembled.append(content)
         next_line = end + 1
 
-    original_line_count = len(
-        (ORIGINAL_STATIC / "app.js").read_text(encoding="utf-8").splitlines()
-    )
-    if next_line != original_line_count + 1:
+    if next_line != AUDITED_FRONTEND_SOURCE_LINE_COUNT + 1:
         raise AssertionError(
-            f"app.js source coverage ended at {next_line - 1}, expected {original_line_count}"
+            "app.js source coverage ended at "
+            f"{next_line - 1}, expected {AUDITED_FRONTEND_SOURCE_LINE_COUNT}"
         )
     return "".join(assembled)