fix: refuse an export without its truth

dist/ is not committed and exports/ is. On a fresh clone, or with a
mistyped --truth-dir, the export copied each platform's file back
uncorrected, said OK and replaced the corrected export in exports/.
check_freshness then reported a stale export instead of the missing
model. The platform now fails, writes nothing, and names the
generate_truth command.

scripts/export_native.py

@@ -457,8 +457,12 @@ def export_platform(
     except StaleTruth as exc:
         return False, [str(exc)]
     if truth is None:
-        truth = {}
-        messages.append(f"no truth for {platform}, only the platform's own data")
+        # Without a truth the export is the platform's file copied back, and
+        # it replaced the corrected one in exports/ on a fresh clone.
+        return False, [
+            f"{truth_dir / f'{platform}.yml'} is absent: run "
+            f"python scripts/generate_truth.py --platform {platform}"
+        ]
 
     systems, report = build_native_model(truth, scraped)
     if not systems: