CSHLD-789: wasm-utxo v6 transparent txns support

[Ranjna-G]

Summary

Zcash's NU7 upgrade deactivates v4 transactions entirely, so transparent-only (t→t)
transfers must also be built, signed, and extracted in v6 wire format. This PR closes
three PCZT-presence assumptions in the v6 PSBT path that previously made a
transparent-only v6 PSBT unsignable/unextractable, all gated behind the existing
require_no_shielded_output_ever_added() helper so shielded (t→z) flows are unaffected.

Linear: CSHLD-789

Changes

• Relax sign_ironwood_v6's ovk-derivation requirement so a transparent-only PSBT
  (no PCZT) can be signed by either key first, not just the user key
• Fix v6_transparent_sighash to compute a bundle-less ZIP-244 digest instead of
  requiring an Ironwood PCZT to exist
• Add extract_transparent_only_v6_tx as the PCZT-free counterpart to
  combine_ironwood_proof, exposed via extractTransparentOnlyTransaction()
• New Rust tests (49 total in zcash_psbt.rs, 627 total in the crate) and a new
  JS/Mocha describe block in test/fixedScript/zcashIronwoodPsbt.ts (64 tests
  in that file, 1575 total repo-wide)

Test Plan

• cargo test --lib — 627 passing, 0 failed
• npx mocha --recursive 'test/**/*.ts' — 1575 passing, 0 failing
• New test: build a transparent-only v6 PSBT, sign with either key first (no
  ovk-order restriction), call extractTransparentOnlyTransaction() directly
  (bypassing combineProof()), assert valid v6 bytes and correct fee accounting
• New test: rejects extraction before the transparent input is signed
• New test: rejects extraction for a PSBT that carries a shielded output

[linear-code]

CSHLD-789

[Ranjna-G]

@claude review

[veetragjain]

@claude

[veetragjain]

Few methods of Ironwood psbt like getId and verifySignature still require a pczt to be present. These methods would throw error for a transparent to transparent transactions.