feat(sdk-coin-starknet): implement Starknet SDK module

[shubham-damkondwar]

Summary

• Implements modules/sdk-coin-starknet — full unique chain SDK package
• Coin class, transaction builders, key pair, utils, and full test suite
• 45 unit tests passing

Chain Type

ECDSA (secp256k1) — uses OZ EthAccountUpgradeable for account abstraction on Starknet L2

MPC Support

• secp256k1 ECDSA TSS (same as ICP)
• shouldHash=false (Poseidon pre-hashed transactions)
• Signature format: [r_low, r_high, s_low, s_high, v]

Reference Implementations

• ICP (coin class skeleton, ECDSA key pair, MPC patterns)
• strkMPC sandbox (transaction building, address derivation, signature formatting)

Changes

• src/starknet.ts + tstarknet.ts — mainnet + testnet coin classes
• src/lib/transactionBuilderFactory.ts — builder factory
• src/lib/transferBuilder.ts — STRK transfer transactions (ERC-20 calldata)
• src/lib/keyPair.ts — secp256k1 key pair with Starknet address derivation
• src/lib/transaction.ts — transaction parse/explain/serialize
• src/lib/utils.ts — address validation, EthAccount address computation, signature formatting
• src/lib/iface.ts, constants.ts — types and chain constants
• test/unit/ — keyPair, transaction, builder, coin, and utils tests

Related

• Jira: CECHO-924
• Depends on feat(statics): add Starknet to statics and environments #8780 (must merge first)
• Part 2 of 3 — factory wiring follows in the next PR

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