Adds tool/check_arb_placeholder_corruption.py, a static check that flags
literal "?" glued to an ICU placeholder brace in any lib/l10n/app_*.arb
value that has a placeholders metadata block. This is the exact corruption
shape fixed in the previous commit; flutter analyze doesn't catch it since
the JSON/ICU stays syntactically valid. Wired as a CI step before
flutter analyze so it fails fast.
Also audited lib/l10n/app_localizations_ext.dart (hand-maintained weekday/
month/date-sentence maps, not covered by ARB tooling): all 22 locale maps
have the full 13/13 keys with no corruption or leftover English — no
changes needed there.
