fix(types): type fastify.jwt as namespace map when namespaces are declared

[xianjianlf2]

Fixes #395 (also addresses the fastify.jwt half of #321).

Problem

When registering the plugin with the namespace option, fastify.jwt becomes a { [namespace]: JWT } map at runtime (index.js), but the type declaration hardcodes FastifyInstance.jwt as JWT. Because TypeScript declaration merging cannot override a conflicting property type, users have no way to correct this on their side — app.jwt.myNamespace.sign(...) always fails to typecheck even though it works at runtime.

Fix

Types-only, no runtime change. fastify.jwt is now typed with a new exported JwtDecorator conditional type. It defaults to JWT, so existing non-namespace users are completely unaffected. Namespace users declare their registered namespaces through the existing FastifyJWT declaration-merging interface (the same mechanism already used for payload/user):

declare module '@fastify/jwt' {
  interface FastifyJWT {
    namespaces: 'auth' | 'admin'
  }
}
// app.jwt.auth.sign(...) / app.jwt.admin.verify(...) now typecheck

The conditional uses [Namespaces] extends [string] to avoid distributing over the union (Record<'auth', JWT> | Record<'admin', JWT> would be wrong).

Note on #397: the registration-scoped types rework will touch types/index.d.ts too, but it is blocked on unmerged fastify core work and explicitly keeps namespaced decorators on declaration-merging helpers, so this interim non-breaking fix is complementary rather than competing.

Tests

New types/namespace.tst.ts asserts the map shape (Record<'auth' | 'admin', JWT>), per-namespace JWT methods, and negatives: the single-decorator members (sign/verify/decode) and undeclared namespaces are not present. tstyche checks test files in isolation, so the augmentation does not affect the existing default-mode assertions in types/index.tst.ts.

README: documented the pattern in the Namespace → TypeScript section.

Checklist

• run npm run test — lint clean, 176/176 unit tests (100% coverage), 35/35 type assertions
• tests are included
• documentation is changed or added
• commit message and code follows the Developer's Certificate of Origin and the Code of Conduct