fix: diff every field a platform rescrape writes

diff_platform compared only version, files and core lists: a rescrape
that changed launchers, dual_mode_cores or a system's core, native_id or
data_directories came out identical. Every other field is now compared,
without the revision read (source) and the source_ref lines that move with it.

scripts/check_freshness.py

@@ -107,6 +107,9 @@ class PlatformDiff:
     files_changed: list[str] = field(default_factory=list)
     cores_added: list[str] = field(default_factory=list)
     cores_removed: list[str] = field(default_factory=list)
+    # Any other field the scrape writes: launchers, dual_mode_cores, a
+    # system's core, native_id or data_directories.
+    fields_changed: list[str] = field(default_factory=list)
 
     @property
     def content_changed(self) -> bool:
@@ -119,6 +122,7 @@ class PlatformDiff:
                 self.files_changed,
                 self.cores_added,
                 self.cores_removed,
+                self.fields_changed,
             )
         )
 
@@ -138,6 +142,7 @@ class PlatformDiff:
             ("~files", self.files_changed),
             ("+cores", self.cores_added),
             ("-cores", self.cores_removed),
+            ("~fields", self.fields_changed),
         ):
             if items:
                 shown = ", ".join(items[:SHOWN_ITEMS]) + (", ..." if len(items) > SHOWN_ITEMS else "")
@@ -177,6 +182,33 @@ def _keyed_files(files: list[dict]) -> dict[str, dict]:
     return keyed
 
 
+# Compared on their own above. `source` is the revision read: it moves with
+# every upstream commit, and what that commit changes is diffed field by field.
+_DIFFED_APART = frozenset({"version", "systems", "cores", "standalone_cores", "source"})
+
+
+def _without_refs(value: object) -> object:
+    """*value* without its `source_ref` lines.
+
+    A launcher cites lines of the revision read, so they shift with any
+    upstream commit above them (es_features.cfg:11638 read as :11649 one
+    RetroBat commit later). The cited content is what is compared.
+    """
+    if isinstance(value, dict):
+        return {k: _without_refs(v) for k, v in value.items() if k != "source_ref"}
+    if isinstance(value, list):
+        return [_without_refs(v) for v in value]
+    return value
+
+
+def _changed_fields(old: dict, new: dict, skip: frozenset[str], prefix: str = "") -> list[str]:
+    return [
+        f"{prefix}{key}"
+        for key in sorted(set(old) | set(new), key=str)
+        if key not in skip and _without_refs(old.get(key)) != _without_refs(new.get(key))
+    ]
+
+
 def diff_platform(old: dict, new: dict) -> PlatformDiff:
     """Compare two platform files the way a reviewer reads the diff.
 
@@ -205,6 +237,11 @@ def diff_platform(old: dict, new: dict) -> PlatformDiff:
             for k in sorted(set(old_files) & set(new_files))
             if old_files[k] != new_files[k]
         )
+        diff.fields_changed.extend(_changed_fields(
+            old_systems[system] or {}, new_systems[system] or {},
+            frozenset({"files"}), f"{system}/",
+        ))
+    diff.fields_changed = _changed_fields(old, new, _DIFFED_APART) + diff.fields_changed
     old_cores, new_cores = _core_names(old), _core_names(new)
     diff.cores_added = sorted(new_cores - old_cores)
     diff.cores_removed = sorted(old_cores - new_cores)

tests/test_check_freshness.py

@@ -165,6 +165,32 @@ class PlatformDiffTests(unittest.TestCase):
         self.assertEqual(diff.cores_removed, ["b"])
         self.assertIn("+cores 2", diff.summary())
 
+    def test_every_other_scraped_field_is_a_change(self):
+        """launchers feed verify, dual_mode_cores the grouping, a system's
+        data_directories the pack: a rescrape that moves only them is stale."""
+        old = self._platform()
+        old["launchers"] = [{"emulator": "e"}]
+        old["dual_mode_cores"] = ["flycast"]
+        old["systems"]["sys"].update(core="c1", native_id="psx",
+                                     data_directories=[{"ref": "x"}])
+        new = self._platform()
+        new["launchers"] = []
+        new["systems"]["sys"].update(core="c2", native_id="psx")
+        diff = cf.diff_platform(old, new)
+        self.assertTrue(diff.content_changed)
+        self.assertEqual(
+            diff.fields_changed,
+            ["dual_mode_cores", "launchers", "sys/core", "sys/data_directories"],
+        )
+        self.assertIn("~fields 4", diff.summary())
+
+    def test_the_revision_read_alone_is_not_a_change(self):
+        old, new = self._platform(), self._platform()
+        old["source"], new["source"] = "https://x/tar.gz/aaa", "https://x/tar.gz/bbb"
+        old["launchers"] = [{"emulator": "e", "source_ref": "es_features.cfg:11638-11646"}]
+        new["launchers"] = [{"emulator": "e", "source_ref": "es_features.cfg:11649-11657"}]
+        self.assertFalse(cf.diff_platform(old, new).changed)
+
     def test_target_stamp_is_not_a_change(self):
         old = {"scraped_at": "2026-01-01", "targets": {"t": {"cores": ["a"]}}}
         new = {"scraped_at": "2026-02-01", "targets": {"t": {"cores": ["a"]}}}