Add check-flavor.py: verify an artifact's flavour without installing it

Reads the Dart AOT snapshot out of an .apk or a Linux/Windows bundle .zip and
asserts exactly one AMBER_FLAVOR:: marker matching the expected variant. See the
amber-app commit for why nothing cheaper distinguishes the two builds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/check-flavor.py

@@ -0,0 +1,80 @@
+#!/usr/bin/env python3
+"""Verify a built artifact's flavour without installing or launching it.
+
+    check-flavor.py <artifact> <expected: adult|clean>
+
+Exits non-zero unless the artifact contains exactly one `AMBER_FLAVOR::` marker
+and it matches. Handles an `.apk`, a Linux bundle `.zip` and a Windows bundle
+`.zip` — it finds the Dart AOT snapshot inside each.
+
+**Why this exists.** The clean flavour is what anonymous downloads and every
+child profile receive, so publishing an adult build into a clean slot is the
+worst mistake this release process can make — and until now the only way to tell
+the two apart offline was to install the APK on a television and look at the tab
+row. That made a content-safety gate depend on a TV being awake and on adb still
+being authorised, which is exactly how it gets skipped "just this once".
+
+Nothing cheaper worked. Class names such as `AdultBrowse` survive in **both**
+snapshots whatever the dart-define, and the `'adult'`/`'clean'` strings the
+updater compares sit inside a function body, so both literals ship in both
+builds. The two APKs are also routinely **byte-identical in size** (zip
+alignment absorbs the difference), so size proves nothing either. `config.dart`
+therefore compiles in a deliberate const-folded marker; see [kFlavorMarker].
+"""
+import re
+import sys
+import zipfile
+
+MARKER = re.compile(rb'AMBER_FLAVOR::(adult|clean)')
+
+# Where the Dart AOT snapshot lives, per artifact kind.
+SNAPSHOTS = (
+    'lib/arm64-v8a/libapp.so',   # apk
+    'lib/armeabi-v7a/libapp.so',  # apk, 32-bit
+    'lib/libapp.so',              # linux bundle zip
+    'data/app.so',                # windows bundle zip
+)
+
+
+def markers_in(blob: bytes) -> set:
+    return {m.group(1).decode() for m in MARKER.finditer(blob)}
+
+
+def main(path: str, expected: str) -> int:
+    if expected not in ('adult', 'clean'):
+        print(f'error: expected must be adult|clean, got {expected!r}')
+        return 2
+
+    found = set()
+    checked = []
+    with zipfile.ZipFile(path) as z:
+        names = set(z.namelist())
+        for candidate in SNAPSHOTS:
+            if candidate in names:
+                checked.append(candidate)
+                found |= markers_in(z.read(candidate))
+
+    if not checked:
+        # Better to fail loudly than to pass an artifact nothing was read from.
+        print(f'FAIL {path}: no Dart snapshot found (looked for {SNAPSHOTS})')
+        return 1
+    if not found:
+        print(f'FAIL {path}: no AMBER_FLAVOR marker in {checked} — '
+              'built before the marker existed?')
+        return 1
+    if len(found) > 1:
+        print(f'FAIL {path}: ambiguous, contains {sorted(found)}')
+        return 1
+
+    actual = found.pop()
+    if actual != expected:
+        print(f'FAIL {path}: is {actual!r}, expected {expected!r}')
+        return 1
+    print(f'ok   {path}: {actual} (from {", ".join(checked)})')
+    return 0
+
+
+if __name__ == '__main__':
+    if len(sys.argv) != 3:
+        raise SystemExit(__doc__)
+    raise SystemExit(main(sys.argv[1], sys.argv[2]))