feat(wasm-utxo): add sighash policy primitives to BitGoPsbt

[ralph-bitgo]

What

• Adds get_input_sighash_types() to the BitGoPsbt wasm class (and the TypeScript wrapper as getInputSighashTypes()): returns each input's declared BIP-174 PSBT_IN_SIGHASH_TYPE, undefined/null when absent (signer default applies).

• Adds assert_sighash_all_policy() (TS: assertSighashAllPolicy()): asserts the policy required when signing externally supplied PSBTs. For every input it rejects

  • a declared sighash type that does not commit to the entire transaction, and
  • any signature already present (ECDSA partial signatures, Taproot key-path/script-path signatures) under such a type.

  Accepted types are SIGHASH_ALL (0x01), SIGHASH_ALL|SIGHASH_FORKID (0x41) on BCH-family coins, and SIGHASH_DEFAULT (0x00)/SIGHASH_ALL on Taproot inputs; an absent type uses the signer default. MuSig2 partial signatures carry no sighash byte and are governed by the declared type checked above.

• Rust unit tests cover the accepted/rejected sets per network (incl. BCH FORKID and Taproot DEFAULT) and the policy assert on declared types, injected signatures, and real signed wallet PSBTs; mocha tests in test/fixedScript/sighashPolicy.ts cover the wasm/TS surface end-to-end, including real signing runs for p2wsh/BCH-p2sh/Taproot and the output-swap invalidation property.

Why

The signer honors the per-input PSBT_IN_SIGHASH_TYPE embedded in a PSBT. A foreign PSBT can request SIGHASH_NONE/SINGLE/ANYONECANPAY, producing a signature that does not bind the signer to the transaction outputs — AnchorWatch demonstrated an output-swap drain of recovered funds this way (SIG-001 / WCN-1994). Consumers that ingest foreign PSBTs need one shared, network-aware primitive; per-app reimplementations miss the BCH FORKID and Taproot rules (a hardcoded 0x01-only check rejects every BCH PSBT).

Test plan

• cargo test --lib — 620 passed (5 new tests)
• cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --check — clean
• tsc --noEmit -p tsconfig.test.json — clean
• Full mocha suite (test/**/*.ts) — 1572 passing (14 new tests)
• CI (test:wasm-pack chrome/node legs run in CI; not runnable in this sandbox)

Stack

This PR is part 1 of 3 in the WCN-1994 stack. Review and merge in order:

1. This PR — wasm-utxo primitives (publishes as @bitgo/wasm-utxo ≥ 5.6.0)
2. BitGoJS @bitgo/abstract-utxo — signExternalPsbt / assertExternalPsbtSighashPolicy built on these primitives (publishes as ≥ 13.3.0)
3. wallet-recovery-wizard PR #753 — WRW "Sign Unsigned PSBT" flow switches to the library helpers

Ticket: WCN-1994

[linear-code]

WCN-1994