Replace the per-provider replay window with a per-owner agreement nonce

Problem

AgreementTerms.nonce is provider-chosen and checked against a per-provider
sliding replay window (ProviderReplayStates: StorageMap<AccountId, ReplayWindow>,
crates/primitives/storage/src/provider_replay_state.rs): a 1024-bit bitmap
anchored at the highest accepted nonce, with NonceAlreadyUsed / NonceTooOld
rejections (#105).

That is more machinery than the problem needs:

• Provider must keep durable nonce state. Reuse a nonce after a restart and
  the quote is rejected; fall more than 1024 behind the newest redeemed quote and
  it is rejected as too old. Neither failure exists for the party actually
  submitting the transaction.
• Ties replay protection to provider lifetime. complete_deregister clears
  ProviderReplayStates, so a quote signed before deregistering could be
  redeemed against a re-registration. Today this is what the
  DeregisterAnnouncementPeriod > RequestTimeout invariant papers over, and it
  is one of the obstacles to a one-step deregister_provider.
• 136 bytes per provider plus bit-shift logic for a check that is
  structurally identical to the system account nonce.

Proposal

Model it on the system account nonce, keyed by the owner (already in the
signed terms; redemption already requires origin == terms.owner):

/// Next expected `AgreementTerms.nonce` per owner.
#[pallet::storage]
pub type AgreementNonces<T: Config> =
    StorageMap<_, Blake2_128Concat, T::AccountId, u64, ValueQuery>;

// in both establish_* paths, after signature verification:
ensure!(terms.nonce == AgreementNonces::<T>::get(&owner), Error::<T>::NonceMismatch);
AgreementNonces::<T>::mutate(&owner, |n| *n = n.saturating_add(1));

Flow: owner reads AgreementNonces[owner], passes it in the quote request, the
provider signs the terms as given. Redemption checks equality and increments.
An owner with several quotes outstanding hands out consecutive nonces and
redeems them in that order, exactly as it would batch transfers.