fix: count a core file filling a declaration

In existence mode the builder packs a core's file at a destination the
platform declares and does not hold, and counts the declaration met.
verify now reads the same placement and reports it OK, so both sides
agree on the platform's count.

scripts/packextras.py

@@ -137,6 +137,7 @@ def hidden_extras(
     shipped: dict[str, str],
     target_cores: set[str] | None = None,
     region_drops: set[str] | None = None,
+    placed: set[str] | None = None,
 ) -> dict[tuple[str, str, str], str]:
     """Report entries the builder holds and still leaves out, with the reason.
 
@@ -150,7 +151,8 @@ def hidden_extras(
     while the report counted it in the pack. A twin carrying the same bytes
     serves the core on the filesystem that folds the names, so it is not a
     file left out. ``shipped`` maps the full destination of each platform
-    file the pack carries to its local copy.
+    file the pack carries to its local copy. ``placed``, when given,
+    receives the full destination of each held extra the pack carries.
     """
     from packpaths import _register_path, placement_block
 
@@ -198,6 +200,8 @@ def hidden_extras(
         content[full.lower()] = held
         if case_insensitive:
             lower.add(full.lower())
+        if placed is not None and local:
+            placed.add(full)
     return hidden