contracts: sign a contract spend only as the wallet showed it

[GracedEternalKingCabbageMan]

Signing for a contract under the five-point rule. A wallet signs a contract spend only through Approval, which checks the rule, shows what is signed, and signs only the digest of what it showed.

What it does

• Approval::prepare(spend, view, signer) checks:

  1. the template hash is on the wallet's list;
  2. the output was recomputed and the coin pays it;
  3. the program was run against the final transaction;
  4. writes the summary of point 4;
  5. the key is a contract key under m/8383h/{coin}h/0h, the one the path names.

  It also checks the chain's locks.

• The summary shows:

  • the template: by registry name, else its commitment root;
  • the path: who can take it and its effect;
  • every parameter by role: ticker, amount in the asset's precision, the wallet's key marked, a relative lock as a delay;
  • the wallet's balance change in every asset, the contract's, the payments and the fee;
  • each check in words, and a digest over the whole summary.

• Approval::sign(shown_digest, signer) refuses any other digest. It prepares the spend again, refuses if anything differs, and runs the program once more against the transaction it returns. A signature made only to run the program before approval is dropped inside the engine.

• Spend::finalize becomes crate-private, so the gate is the only way to sign.

• lwk_wasm: ContractApproval.prepare(spend, signer, viewJson), .summary(), .digest(); Signer.signContractSpend(approval, shownDigest).

Proof (node v25.2.1, elementsregtest, Simplicity from genesis, -par=1). Before this change the new tests do not compile (unresolved import lwk_contracts::approval), and the wasm drive fails at ContractApproval undefined.

• cargo test -p lwk_contracts --test approval, no chain needed:
  • the summary shows root c1a71ea2…46fa unregistered, or sequentia/one-key v1 (registered);
  • a digest that was not shown, another wallet, a template not on the list, a key outside the contract account, and the wrong contract key are each refused.
• --test drip_regtest through the gate: the drip confirms; the three bad drips are refused by the engine and, forced, in blocks (bad-txns-nonfinal; Assertion failed inside jet twice).
• lwk_wasm/tests/node/contract_drip_regtest.js, the wallet flow in wasm:
  • the approval shows sequentia/faucet-drip v1 (registered), all 13 parameters by role, and the wallet's change 500.00000000 tSEQ;
  • drip 1 confirms at 581 vB, exactly the vsize shown;
  • a digest that was not shown, another wallet, a template not on the list and a non-contract key are each refused;
  • the early drip is refused by the engine; signed by an engine told the interval had passed, it is refused by the mempool (non-BIP68-final) and in a block (bad-txns-nonfinal);
  • the over-tier drip and the wrong successor are each refused by the engine, naming the failing check;
  • drip 2 confirms.