types!: port flash to registration-scoped typing model

[mcollina]

Summary

Port @fastify/flash to the registration-scoped typing approach and remove
migration crutches in final validation.

Highlights:

• plugin typing updated to model request/reply decorator effects from
  registration;
• flash internals tightened (unknown/guards instead of loose typing);
• tests updated to use explicit local narrowing helpers;
• final state validated without test compat bridge and without skipLibCheck.

Dependency

This PR depends on Fastify core typing changes:

• types!: remove declaration-merging reliance with registration-scoped decorator typing fastify#6581 (feat/typed-decorators)
• types!: add createPlugin helper for registration-scoped typing fastify-plugin#282 (feat/create-plugin)

Validation

• npm test ✅
• npm run build ✅

Context

Refs fastify/fastify#5061.