Btc flow

[Pessina]

Adds full Bitcoin custody flow alongside the existing ERC-20 bridge. Key updates:

• Implement dedicated BTC contexts, state, and instruction handlers for deposit, claim, withdrawal, and completion, plus shared config management.
• Refresh TS client stack (test runner settings, BN handling, fakenet-signer path) and extend integration tests with a Bitcoin deposit suite.
• Expand environment/docs to cover Bitcoin network setup (regtest/testnet/mainnet), new .env options, and regtest tooling.

Follow up:

• Move aggregated MPC signature subscriber into signet.js #18
• Add unhappy-path tests for BTC vault flows #19