feat: validate local RPC targets independently of transports

[irvinebroque]

Validation currently attaches to a class decorator or a transport-specific response helper. That makes it awkward to take an existing object and expose only a chosen interface through another Cap'n Web transport.

validateTarget<PublicApi>(target) adds that missing step: validate the object first, then give it to the transport. It reuses the existing server wrapper to reject undeclared methods and invalid arguments, including on returned capabilities. The interface is inferred when no type argument is supplied.

The README and validation guide show usage and explain that native Workers RPC still needs @validateRpc(), because it requires branded objects rather than proxies. A minor changeset is included.

Validation: 617 Node and Workers tests, including a real MessagePort session, plus build, type checks, and Markdown lint pass.

[github-actions]

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