fix: share nested RPC validators within a module

[irvinebroque]

An API can return several objects with the same interface. Today, the validation compiler copies their rules into every place they are used, making generated bundles unnecessarily large.

This patch emits each distinct service validator once per module and reuses it. Server and client rules stay separate. Sharing compares the actual referenced type definitions, so two services called Shared cannot accidentally share string checks when one expects numbers.

Registration and emission live together in one focused module. Six regression tests cover reuse, recursive references, different contracts, and runtime rejection of invalid values. All 617 Node and Workers tests, build, type checks, and Markdown lint pass.

The package README, validation guide, and patch changeset are updated. No application changes are required.

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