feat: mark files a core finds by content

DuckStation identifies its BIOS by MD5 in a folder it scans
(bios.cpp:208), so its entries' names are labels. The export wrote them
into Batocera's and ROCKNIX's lists as files to hold: a user with only
scph5501.bin read 104 missing BIOS where DuckStation boots. Labels also
lent their md5 to a name another core opens, and paired by name they
replaced System.dat's scph5000.bin hashes.

found_by: name | content, on a profile or a file and keyed by build like
load_from. The truth records which cores open each name (_named_by). A
label pairs with a platform entry by hash only, is never added, merges
with an opened name only on a shared hash, and is counted in the export
report rather than listed as missing.

scripts/export_native.py

@@ -612,6 +612,10 @@ def export_platform(
     withheld = exporter.withheld()
     if withheld:
         summary += f", {_count(len(withheld), 'correction')} withheld"
+    if report.labels:
+        # A content-scanning core's names are labels: a list checked by path
+        # cannot hold them.
+        summary += f", {_count(report.labels, 'entry', 'entries')} found by content not listed"
     messages.append(summary)
     messages.extend(f"withheld: {reason}" for reason in withheld)
     for correction in applied[:5]:
@@ -653,6 +657,7 @@ def export_platform(
         "hashes_cleared": cleared,
         "requirements_corrected": requirements,
         "withheld": list(withheld),
+        "found_by_content": report.labels,
         "invalid": issues,
     })
     return True, messages