capnweb-validate: validateRpc() in static block, not just a decorator

[teamchong]

Decorators don't survive Vite 8 / oxc unless experimentalDecorators is on, and
turning that on isn't free: it switches TypeScript to legacy decorator semantics,
which breaks anything already using standard decorators
(cloudflare/workers-sdk#12626). So validation shouldn't require a decorator.

Before:

@validateRpc()
export class Api extends RpcTarget {
  async authenticate(token: string): Promise<User> { ... }
}

After:

export class Api extends RpcTarget {
  static { validateRpc(); }

  async authenticate(token: string): Promise<User> { ... }
}

Each form emits the same validator as the decorator it replaces, and a module
using both shares one. A static block runs when the class is created and isn't
inherited, so the validator is per-class, same as the decorator.

Forms

Form	Same as
static { validateRpc(); }	@validateRpc()
static { validateRpc<Surface>(); }	@validateRpc<Surface>()
static { validateRpc({ skip: ["raw"] }); }	@skipRpcValidation() on raw()
static { validateRpc<Surface>({ skip: ["raw"] }); }	both at once
export default validateRpc(Api);	@validateRpc(), for a class exported as an expression

Build errors

Rule	Why
A call whose result is discarded must be a static block in the class it validates	Anywhere else the call is detached from the class, and only validates it if that code runs
validateRpc() with no class is only valid in a static block of a named class declaration	That is the only place the class it validates is unambiguous, and has a name to emit
Surface goes through the factory form, not validateRpc<Surface>(Api)	TypeScript has no partial type-argument inference, so the class type would be discarded
The argument must name a class declared in the same module, not an import or inline class { ... }	The transform reads the declaration for @skipRpcValidation() members and platform method filtering
skip must be an inline object literal with an array of string literals	The names are read at build time
A skip name must exist in the resolved surface, and isn't allowed twice; with a type argument it's keyof Surface too	Same as a stray @skipRpcValidation(); a repeat is always a mistake

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commit: 615cca1