migration: audit uncertain code and prepare handoff

app/tests/preservation_helpers.py

@@ -17,6 +17,16 @@ SOURCE_RANGE = re.compile(
     re.DOTALL,
 )
 
+# These exact original app.js line ranges were retired in slice 11 after the
+# definition-only symbols passed static, runtime, and compatibility review.
+RETIRED_FRONTEND_SOURCE_RANGES = (
+    (3537, 3540),
+    (4139, 4145),
+    (4973, 4989),
+    (9022, 9027),
+    (9052, 9055),
+)
+
 
 def sha256(path: Path) -> str:
     return hashlib.sha256(path.read_bytes()).hexdigest()
@@ -38,8 +48,6 @@ def reassembled_frontend_runtime() -> str:
             raise AssertionError(
                 f"app.js source coverage gap: expected line {next_line}, got {start}"
             )
-        if len(content.splitlines(keepends=True)) != end - start + 1:
-            raise AssertionError(f"app.js line count changed in range {start}-{end}")
         assembled.append(content)
         next_line = end + 1
 
@@ -53,6 +61,27 @@ def reassembled_frontend_runtime() -> str:
     return "".join(assembled)
 
 
+def original_runtime_after_audited_retirements() -> str:
+    lines = (ORIGINAL_STATIC / "app.js").read_text(encoding="utf-8").splitlines(
+        keepends=True
+    )
+    retired = {
+        line_number
+        for start, end in RETIRED_FRONTEND_SOURCE_RANGES
+        for line_number in range(start, end + 1)
+    }
+    return "".join(
+        line for line_number, line in enumerate(lines, start=1) if line_number not in retired
+    )
+
+
+def assert_frontend_runtime_matches_audited_baseline(testcase) -> None:
+    testcase.assertEqual(
+        reassembled_frontend_runtime(),
+        original_runtime_after_audited_retirements(),
+    )
+
+
 def assert_moved_asset_matches(
     testcase,
     original_relative: str,