feat(zcash-wasm): multisig verifier primitives

[Catopish]

Summary

• New frost_parse_tx_outputs(unsigned_tx_hex, ufvk) WASM export. Parses a v5 tx, OVK-decrypts orchard outputs (external + internal scopes), recomputes ZIP-244 sighash for end-to-end verification of joiner sign sessions.
• build_unsigned_transaction + build_signed_spend_transaction bind out_ciphertext to the wallet's FVK OVK instead of a random ephemeral key. Required so the spender's quorum (FROST cosigners) can decrypt their own outputs; matches zcashd default.
• v5 parser accepts NU6.1 branch ID by patching the field in-memory pre-parse (zcash_primitives 0.21 ships zcash_protocol 0.4 which tops out at NU6).

Test plan

• cargo wasm-parallel + cargo wasm-single produce working artifacts
• zafu integration: cosigner sees green ✓ on a legit sign; red mismatch on tampered SIGN: payload
• Existing frost-spend tests still pass

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