Indexer-service no longer verifies RCA proposals via EIP-712 signature
recovery and escrow-authorized signer lookup. Instead, it validates
that an on-chain RCA offer exists by querying the
graphprotocol/indexing-payments-subgraph for the corresponding Offer
entity and comparing the stored offerHash to the locally-computed
hashRCA(rca).

Changes:
- New crates/dips/src/offers.rs module with OfferLookup trait,
  HttpOfferLookup production impl (queries subgraph via raw GraphQL
  POST), and MockOfferLookup test impl with a preload-friendly API.
- Deleted crates/dips/src/signers.rs (SignerValidator trait,
  EscrowSignerValidator wrapper, Noop/RejectingSignerValidator stubs).
  The underlying EscrowAccountsWatcher in crates/monitor stays in
  place because TAP still uses it for payer-signer authorization.
- SignedRecurringCollectionAgreement::validate is now async and takes
  &Arc<dyn OfferLookup> instead of &Arc<dyn SignerValidator>. On an
  empty signature wire payload, it skips signature recovery entirely
  and branches on offer_lookup.get_offer_hash(agreement_id) results.
- DipsError drops SignerNotAuthorised and adds OfferNotFound,
  OfferMismatch, and OfferLookupFailed. reject_reason_from_error
  maps all three to the existing gRPC RejectReason::SignerNotAuthorised
  variant for wire-protocol compatibility -- dipper's response handler
  only distinguishes Accept vs Reject, not between individual Reject
  reasons, so no proto change is needed in this release.
- DipsServerContext swaps the signer_validator field for offer_lookup.
- DipsConfig gains indexing_payments_subgraph_url (required, Url type)
  and service.rs plumbs it into HttpOfferLookup on startup.
- crates/config/maximal-config-example.toml gains the new field and
  the test_maximal_config fixture is updated to match.
- All 57 indexer-dips unit tests updated to use MockOfferLookup with
  pre-seeded (agreement_id -> hash) pairs. Added two new tests:
  test_validate_and_create_rca_offer_not_found (empty lookup) and
  test_validate_and_create_rca_offer_mismatch (seeded with wrong hash).
- The old test_validate_and_create_rca_unauthorized_signer test is
  removed (replaced by the two offer-path tests above).

The wire-level ABI encoding is unchanged: dipper sends a
SignedRecurringCollectionAgreement wrapper with the bytes signature
field present but empty. The contract takes the offer-path branch in
_requireAuthorization when the indexer-agent forwards this payload to
SubgraphService.acceptIndexingAgreement.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

encode_empty_sig was the test-only helper for building RCA payloads
during the offer-based authorisation iteration, where the empty-sig
path meant something. After the revert to proposal-first validation
the validator does not consume the signature at all, so the name
implies a distinction that no longer exists. Rename to
rca_to_wire_bytes, which describes what the helper produces without
suggesting anything about how the payload will be validated.

Add test_rca_conditions_field_roundtrip guarding the sol! struct's
uint16 conditions field against layout drift: encode with a non-zero
conditions value, decode, assert the value survives and that the
neighbouring maxSecondsPerCollection and nonce slots are intact so a
future misalignment is not misread from an adjacent field. This is
the field audited into the audit-branch contract, so a silent decode
drift would produce RCAs whose hash diverges from the contract's view.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>