feat: lower validation decorators to ordinary JavaScript

[irvinebroque]

@validateRpc() tells our build step to add checks to a class. Today, that step leaves a decorator in its output, so the next build tool still needs decorator support. This causes trouble with toolchains such as Vite 8/Oxc.

The transform now removes its markers and makes an ordinary function call after the class declaration. It updates the existing prototype without replacing the class. Named default exports, private fields, inheritance, and static initialization keep working. All recognized skip markers are removed too.

Anonymous classes need names; repeated validation class markers and other class decorators together with @validateRpc produce build errors. The README, validation guide, and minor changeset explain migration.

Validation: 621 Node and Workers tests, build, type checks, and Markdown lint pass.

Upstream #231 tackles the same build-tool problem with different source syntax. Coordinate these approaches before upstreaming.

[github-actions]

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