Fail CI when the manual shows a picture no scene makes

The traceability job now runs `tools/manual/record.sh --check`: every
picture docs/manual/README.md shows must be made by a scene in
tools/manual/scenes.py, and every picture a scene makes must be shown.
It reads the two files and nothing else, so it needs no app, display or
LFS pull.

--changed now dates a scene by the newest commit among its pictures
rather than each picture alone. A scene that also makes a picture which
re-records byte for byte (panorama-aligned beside panorama.gif) no
longer stays listed for ever. A scene all of whose pictures come out
identical (launch) stays listed until one differs, which costs one
harmless re-run.

tools/manual/scenes.py

@@ -1197,28 +1197,28 @@ def own_code_changed(s, since):
 
 
 def changed():
-    """The scenes whose pictures are older than their sources: for each
-    picture, the commit that last touched it, and whether any source — or
+    """The scenes whose pictures are older than their sources: the commit
+    that last touched any of a scene's pictures, and whether a source — or
     the scene's own code — has changed since (the working tree included).
-    A picture never committed is new, and so is its scene."""
+    A scene none of whose pictures was ever committed is new.
+
+    A re-recording that comes out byte-identical touches no picture, so its
+    scene stays listed until one of its pictures really changes; re-running
+    it is harmless."""
     out = []
     for s in SCENES.values():
-        pics = [m for m in s.media if '.' in m]
+        pics = [f'{MEDIA}/{m}' for m in s.media if '.' in m]
         if not pics:
             continue
-        reasons = []
-        for m in pics:
-            since = git('log', '-1', '--format=%H', '--', f'{MEDIA}/{m}').strip()
-            if not since:
-                reasons.append(f'media/{m} was never committed')
-                continue
-            paths = git('diff', '--name-only', since, '--', *[f':(glob){g}' for g in s.sources]).split()
-            if paths:
-                reasons.append(f'{", ".join(sorted(set(paths))[:3])} changed since media/{m}')
-            elif own_code_changed(s, since):
-                reasons.append(f'its scene changed since media/{m}')
-        if reasons:
-            out.append((s.name, reasons[0]))
+        since = git('log', '-1', '--format=%H', '--', *pics).strip()
+        if not since:
+            out.append((s.name, 'its pictures were never committed'))
+            continue
+        paths = git('diff', '--name-only', since, '--', *[f':(glob){g}' for g in s.sources]).split()
+        if paths:
+            out.append((s.name, f'{", ".join(sorted(set(paths))[:3])} changed since {since[:9]}'))
+        elif own_code_changed(s, since):
+            out.append((s.name, f'its scene changed since {since[:9]}'))
     return out