types!: migrate static decorators to registration-scoped typing

[mcollina]

Summary

Migrate @fastify/static typing from global FastifyReply declaration
augmentation to registration-scoped decorator inference.

Changes include:

• types/index.d.ts now models reply decorators as plugin-applied effects;
• types/index.test-d.ts rewritten to assert decorator availability only on
  registered instances;
• README TypeScript notes updated to document the new registration-scoped
  behavior.

Dependency

This PR is part of the declaration-merging removal stack and depends on:

• types!: remove declaration-merging reliance with registration-scoped decorator typing fastify#6581 (feat/typed-decorators)

Please review/merge with that core change in mind.

Validation

• npm run test:typescript ✅
• npm run lint -- types ✅

Context

Refs fastify/fastify#5061.

[Fdawgs]

Do the inline type tags also need updating as part of this?

[jean-michelet]

This would immediately break every application that relies on autoloading, as well as potentially many other patterns people are using in the wild. In large applications, the required refactoring could be significant.

I am not convinced we can ever fully remove declaration merging. It will likely need to remain supported indefinitely.

But there might be other options.