feat(escrow_wip): build_unsigned_escrow_agree builder
Plutus V3 spend that flips state Open → Agreed { agreed_at_ms = upper }.
Both party_a and party_b sign — driver returns partially-signed CBOR,
co-signer adds witness via wallet_sign_partial, signed tx submitted.
Validator enforces validity_upper ≤ open_deadline_ms (preflighted),
value preserved bit-exact, deposits preserved bit-exact via cbor.serialise
equality (passes naturally since we copy the Vec verbatim and
to_plutus_data emits deterministic KeyValuePairs).
7 tests cover negative paths (non-Open state, outsider driver,
post-deadline upper) and happy paths (a-drives, b-drives, datum
state flips correctly, immutable fields + deposits preserved).
415c099240