Add runtime Buffer polyfill checks to prevent initialization order issues

[Copilot]

The Buffer polyfill setup assumed modules would be imported in a specific order. If Buffer.from() was called before the polyfill initialized globalThis.Buffer, operations would fail.

Changes

• Created src/lib/buffer-polyfill.ts: Centralized utility that ensures Buffer is available and exports ensureBuffer() with defensive checks for edge cases (dynamic imports, circular dependencies)

• Updated crypto modules: bitcoin.ts, evm.ts, and keyring.ts now call ensureBuffer() once at module load time and cache the result. All Buffer.from() calls use the cached instance.

• Removed unused import: transaction.ts doesn't use Buffer, so removed the unnecessary import.

Example

Before:

// bitcoin.ts - vulnerable to initialization order
import { Buffer } from 'buffer';
if (typeof globalThis.Buffer === 'undefined') {
  globalThis.Buffer = Buffer;
}
// Later in function...
const hex = Buffer.from(data).toString('hex'); // Fails if imported before polyfill runs

After:

// bitcoin.ts - guaranteed availability
import { ensureBuffer } from '../buffer-polyfill';
const BufferImpl = ensureBuffer(); // Once at module level
// Later in function...
const hex = BufferImpl.from(data).toString('hex'); // Always works

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