feat: scoped-unfreeze by reason bitmask (#607) - #699

- Add #[repr(u32)] to FreezeReason with explicit discriminants and
  to_bitmask() helper for safe bitmask operations
- Introduce DataKey2::HolderFreezeMask storing a u32 bitmask instead
  of a single FreezeReason, with legacy EmergencyFreeze fallback on read
- emergency_freeze_holder now ORs reason bits into the mask;
  idempotent when the reason is already active
- Add clear_freeze_reason(reason) to clear a single reason bit;
  only fully unfreezes (removes key, emits frz_clr) when mask == 0;
  emits frz_rc (freeze_reason_cleared) for partial clears
- emergency_unfreeze_holder delegates to clear_freeze_reason for
  backward compatibility
- process_ofac_attestation ORs SanctionsMatch bit with legacy migration
- Add get_holder_freeze_reasons query returning the raw bitmask
- Update storage layout schema with HolderFreezeMask and pre-existing
  missing registrations
- Add 10 comprehensive tests covering multi-reason freeze/clear,
  idempotency, event emission, and OFAC+manual interaction