fix: check core files where the builder puts them

The pack integrity check rebuilt the core extras with no taken paths and
tested presence alone, so it credited core files at paths the platform's
own file holds and printed a core count the builder contradicted. It now
replays the builder's placement through packextras.hidden_extras and
compares each placed member with the copy the builder resolves.

scripts/packextras.py

@@ -216,6 +216,7 @@ def hidden_extras(
     report: list[dict] | None = None,
     resolver: ExtraResolver | None = None,
     placed_kind: dict[str, str] | None = None,
+    placed_local: dict[str, str] | None = None,
 ) -> dict[tuple[str, str, str], str]:
     """Report entries the builder holds and still leaves out, with the reason.
 
@@ -234,7 +235,9 @@ def hidden_extras(
     receives the full destination of each held extra the pack carries, and
     ``placed_kind`` its category, carried by the extra itself: the second
     pass, the archive-prefix pass and the agnostic scan copy files no gap
-    report entry names. ``resolver`` is the builder's lookup. An extra it cannot resolve is
+    report entry names. ``placed_local`` receives the local copy the builder
+    writes at each of those destinations, for the pack integrity check.
+    ``resolver`` is the builder's lookup. An extra it cannot resolve is
     skipped and takes no path, as the builder skips it.
     """
     from packpaths import _register_path, placement_block
@@ -292,6 +295,8 @@ def hidden_extras(
             placed.add(full)
         if placed_kind is not None and local:
             placed_kind[full] = fe.get("category", "bios")
+        if placed_local is not None and local:
+            placed_local[full] = local
     return hidden