refactor: extract APDU building functions as pure utilities

[tomkp]

Summary

Extract inline APDU command building logic into reusable pure functions.

Changes

• Add buildGetResponseCommand(bytesAvailable): Build GET RESPONSE APDU
• Add correctLeInCommand(command, newLe): Correct Le value in command
• Both functions exported from main module
• Add JSDoc documentation
• Add 7 unit tests
• Refactor transmitWithAutoResponse to use the new functions

Example usage

import { buildGetResponseCommand, correctLeInCommand } from 'smartcard';

// Build GET RESPONSE command
const getResponse = buildGetResponseCommand(0x1c); // 00 C0 00 00 1C

// Correct Le in existing command  
const corrected = correctLeInCommand(Buffer.from([0x00, 0xb0, 0x00, 0x00, 0xff]), 0x20);

Test plan

• All 99 unit tests pass
• Lint passes
• Existing transmitWithAutoResponse tests still pass

Fixes #98