feat: logging and event hooks for observability #12

Context

In production, developers need visibility into what BurnerGuard is doing — how many emails are being checked, what's matching, how often the allowlist overrides. The library should provide hooks for consumers to plug in their own logging, metrics, or alerting systems without BurnerGuard depending on any specific logging framework.

Proposed API

Event hooks at creation

const guard = await BurnerGuard.create({
    onVerify: (result) => {
        logger.info('BurnerGuard verify', {domain: result.domain, isMatch: result.isMatch});
    },
    onMatch: (result) => {
        logger.warn('Disposable email detected', {domain: result.domain, matchedOn: result.matchedOn});
    },
    onError: (error, context) => {
        logger.error('BurnerGuard error', {error: error.message, context});
    }
});

Available hooks

Design principles

• No built-in logger — zero dependencies means no winston, pino, bunyan
• Hooks are optional — if not provided, the library is silent
• Hooks are fire-and-forget — errors in hooks don't affect verify() results
• Hooks receive readonly data — consumers can't mutate results through hooks

Acceptance criteria

• Hook options in BurnerGuardOptions
• Hooks fire at the correct points in the verify lifecycle
• Hook errors are swallowed (don't break verify())
• Tests covering: each hook fires correctly, hook errors don't propagate
• README section showing integration with common loggers (pino, winston)

Priority: 🟢 Nice-to-have — polish for production use