Problem:
The previous signing API had several issues:
1. `signInput(index, key)` claimed to sign only one input but actually
   signed ALL inputs internally (for ECDSA), which was misleading
2. MuSig2 bulk signing was inefficient - it recreated SighashCache for
   each input, recomputing sha_prevouts, sha_amounts, sha_scriptpubkeys,
   sha_sequences, and sha_outputs redundantly
3. The API didn't clearly communicate performance characteristics

Solution:
Implement an honest, clean WASM/Rust API that explicitly communicates
what each method does and its performance implications.

New WASM API (backward compatible - old methods kept as deprecated):
- is_musig2_input(index) - check if input is MuSig2 keypath
- sign_all_wallet_inputs(xpriv) - bulk sign ECDSA/Schnorr script path
- sign_wallet_input(index, xpriv) - single input with save/restore
  (documented as NOT faster than bulk)
- sign_musig2_input(index, xpriv) - single MuSig2 input
- sign_all_musig2_inputs(xpriv) - NEW: bulk MuSig2 with SighashCache
  reuse for 2x performance improvement
- sign_replay_protection_inputs(ecpair) - bulk replay protection

TypeScript changes:
- Added isBIP32Arg() type guard to js/bip32.ts
- Updated sign(key) to call both sign_all_wallet_inputs and
  sign_all_musig2_inputs for optimal bulk performance
- Updated signInput() to route to appropriate method based on input type

Rust changes:
- Added sign_with_first_round_and_cache() to Musig2Context that accepts
  external SighashCache and prevouts for efficient batch MuSig2 signing
- Added corresponding wrapper in BitGoPsbt

Performance results (1000 inputs):
- ECDSA bulk: ~1.7ms per input (unchanged)
- MuSig2 bulk: ~4.5ms per input (was ~9.5ms - 2x improvement!)

The MuSig2 improvement comes from reusing SighashCache which caches
the shared BIP-341 sighash components across all inputs.

Issue: BTC-2980