ticket: cecho-1071

Summary
This PR implements full support for the Kaspa (KASPA) coin in the BitGoJS SDK, including transaction building, per-input Schnorr signing, PSKT (Partially Signed Kaspa Transaction) support, and comprehensive test coverage.

Key changes
PSKT — Kaspa's native PSBT equivalent (src/lib/pskt.ts, 590 lines new)

• Implements PSKT from scratch in TypeScript with zero new dependencies, matching the camelCase JSON wire format of rusty-kaspa's wallet/pskt crate
• Full role state machine: CREATOR → UPDATER → SIGNER → COMBINER → FINALIZER → EXTRACTOR
• sign(privateKey) — direct single-key path; computes a distinct keyed Blake2b-256 sighash per input (Kaspa's BIP-143-like per-input scheme)
• addSignature(index, pubKey, sig) — HSM / TSS external signing path; the unsigned PSKT JSON is the only artifact that needs to travel to the signer and back
• combine(other) — Combiner role merges partialSigs from independent signers without conflicts
  finalize() — Finalizer promotes partialSigs → finalScriptSig with Kaspa's push-only opcode (0x41 + 64-byte Schnorr sig + sighash type)
• extract() — Extractor emits the exact broadcast JSON accepted by the Kaspa node REST API
• serialize() / Pskt.deserialize() — JSON round-trip for hand-off between coordinator and HSM/TSS service

Sighash (src/lib/sighash.ts)

• Rewrote to match rusty-kaspa's authoritative implementation: all intermediate and final hashes use keyed Blake2b-256 with key "TransactionSigningHash" — verified to produce signatures that the Kaspa testnet node accepts

Transaction (src/lib/transaction.ts)

• Per-input Schnorr signing loop in sign() with push-only signatureScript layout (0x41 + 64-byte sig + sighash type)
• addSignatureForInput() for the TSS multi-input path
• signablePayloads getter returning one sighash Buffer per input for MPCv2 (DKLS) sessions
• toPskt() / Transaction.fromPskt() bridge methods to/from PSKT

TransactionBuilder (src/lib/transactionBuilder.ts)

• toPskt() — builds the transaction and returns it as an unsigned PSKT at UPDATER role, ready for HSM hand-off

Tests

• test/unit/pskt.test.ts — 27 new tests covering full role flow, serialize/deserialize round-trip, combine() conflict detection, addSignature() HSM path, and verifying that PSKT extract() output is byte-identical to Transaction.toBroadcastFormat()
• test/unit/coin.test.ts — signTransaction tests covering direct private key path (Path A), per-input TSS signatures path (Path B), partial signing returning halfSigned
• Extended transactionFlow.test.ts and transaction.test.ts with multi-input signing and per-input Schnorr verification