feat: embed network_id in signed attestations

[sadiqolalere41-pro]

Closes #578

Changes

src/lib.rs

• Add NetworkIdMismatch = 62 to RevoraError — returned when an attestation's embedded network_id does not match the current chain.
• Add InvalidTransferParticipants = 63 — returned on self-transfer.
• Add SignedAttestation struct with network_id: BytesN<32> (domain separator) and digest: BytesN<32> (pre-signed canonical hash).
• Rewrite transfer_with_attestation: add dual-party auth (from + issuer), self-transfer guard, offering-level freeze check, whitelist check, and all 10 ordered guards documented in the function doc-comment.
• Add compute_attestation_digest (read-only) — returns the canonical sha256(network_id || XDR(issuer) || XDR(namespace) || XDR(token) || XDR(from) || XDR(to) || u32be(amount_bps)) for the current chain.
• Add verify_attestation_digest (read-only) — pre-flight check that rejects attestations whose network_id or digest does not match. Returns Err(NetworkIdMismatch) for both mismatch cases (fail-closed design).
• Add private build_attestation_digest helper used by both public fns.

src/test_transfer_with_attestation.rs

• Update module doc-comment to include network-id test coverage table.
• Add make_signed_attestation test helper.
• Add 6 network-id domain separator tests:
  • verify_attestation_correct_network_id — golden path
  • verify_attestation_mainnet_id_on_testnet_rejected
  • verify_attestation_testnet_id_on_mainnet_rejected
  • verify_attestation_unknown_network_id_rejected
  • verify_attestation_wrong_digest_rejected
  • attestation_compute_verify_round_trip (no-aliasing property)

docs/transfer-with-attestation.md

• Document SignedAttestation struct and its two fields.
• Document compute_attestation_digest and verify_attestation_digest.
• Add digest-construction preimage spec (network_id || XDR fields).
• Add security guarantees table (replay prevention, parameter binding, no-aliasing, read-only verification, fail-closed behaviour).
• Add NetworkIdMismatch to the error reference table (code 62).
• Add full test-coverage table for all 24 test cases.

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