feat: expose webhook helpers via ./webhooks subpath export (Closes #410)

[Zhiyilang074811]

Summary

Publishes the existing webhook signature helpers through a ./webhooks\ subpath, making \�erifyWebhookSignature, \�erifyWebhookJSON, \parseWebhookHeader, and \�erifyWebhookWithReplay\ available to consumers.

Changes

• tsup.config.ts: Added \src/webhooks.ts\ to the Node build entry points
• package.json: Added ./webhooks\ export entry with import/CJS/type conditions
• src/index.ts: Added root re-exports for all four webhook helpers
• test/dist-smoke.mjs: Added runtime checks for all four exports in both ESM and CJS
• tests/api-report-snapshot.test.ts: Added \webhooks.d.ts\ to the subpath .d.ts existence check
• tests/exports-subpath-resolves.test.ts: Added ./webhooks\ to the expected subpaths assertion

Acceptance Criteria

• \import { verifyWebhookSignature } from '@lily-protocol/sdk/webhooks'\ resolves against the built package (verified by exports-subpath-resolves test)
• The four helpers appear in the dist smoke test and will ship as \webhooks.d.ts\ in the package
• Root re-exports ensure they're also accessible from the main entry point

Closes #410

[Zhiyilang074811]

Hi! Friendly ping — open for a couple of days, all CI checks passing. Ready for review whenever convenient. Thanks! 🙏

[Zhiyilang074811]

Hi! Friendly ping — bounty PR (#410) open for ~2 days, all checks passing. Ready for review. Thanks! 🙏