fix(wasm-utxo): reject duplicate wallet role keys

[ralph-bitgo]

What

• RootWalletKeys::new / new_with_derivation_prefixes are now fallible and reject repeated role xpubs with a typed WalletKeyError::DuplicateRootKeys, surfaced to JS/WASM callers as a branchable code: "WalletKeyError.DuplicateRootKeys" error instead of a StringError.
• Added require_distinct_pubkeys and enforced it at the shared sink WalletScripts::new and in every script family: build_multisig_script_2_of_3 (builder and parser), ScriptP2tr::new, ScriptP2mr::new, build_tap_tree_for_output, create_tap_bip32_derivation_for_output. The second, derived-key check protects custom derivation prefixes and any future constructor that bypasses the root check.
• Removed the Taproot expect calls: BitGoMusigError (e.g. InvalidPubkeyCount for identical user/BitGo keys) is now a typed WasmUtxoError::BitGoMusig propagated through ScriptP2tr::new and the PSBT/BIP-322 callsites, instead of a panic that traps the whole WASM instance.
• Regression coverage: all three duplicate-pair permutations plus the all-equal triple, default and custom derivation prefixes, every output script family, both Taproot aggregation modes, and both the TypeScript wrapper and raw WASM constructors (including a post-failure call proving the module stays usable).

Why

A wallet threshold counts distinct authorization principals, but nothing in this library enforced that (AnchorWatch finding csf_be4ee1b1031cad5e945939bf2, WCN-1863). With the same xpub in two roles, the legacy builders emitted a nominal 2-of-3 OP_CHECKMULTISIG script [A,A,B]; Bitcoin consensus accepts one valid signature copied into two stack positions, so a single key holder could spend unilaterally once a victim funded an address derived from the malicious triple. On Taproot chains the duplicate user/BitGo key reached expect("valid aggregation") and aborted the entire WASM instance rather than returning a typed error. This closes the quorum-collapse path fail-closed at the root-key boundary and again on derived keys immediately before script assembly.

Test plan

• CI: cargo test + cargo clippy + cargo fmt (wasm-utxo)
• CI: npm run build:test && npm run test:mocha (wasm-utxo), including test/fixedScript/walletKeys.ts
• Review: duplicate-role rejections typed as WalletKeyError.DuplicateRootKeys / DuplicateDerivedKeys, not string errors
• Review: Taproot duplicate user/BitGo keys return BitGoMusigError.InvalidPubkeyCount instead of panicking

Verification note: this sandbox has no cargo/rustc, no Nix dev shell, and no node_modules/generated WASM bindings, so the suites could not be executed locally before opening this draft. The branch CI is the verification gate — do not mark ready for review until the above checks are green.

Ticket: WCN-1863

[linear-code]

WCN-1863