perf: build transport encoders once instead of on every response

[Nic-Polumeyv]

The encode map derived from the transport hook was rebuilt on every serialized response, in server_data_serializer_json (every __data.json request, plus SSR page renders that stream data) and in stringify_action_response (every form action JSON response). The map only depends on the transport hook, which is fixed at server init, and init already builds the mirror decoders map exactly once in runtime/server/index.js. This moves the encode map into a get_encoders helper next to create_replacer, memoized per transport object with a WeakMap so dev-mode re-inits stay safe.

The uneval paths are untouched since create_replacer already iterates the transport lazily. The per-request spread in server_data_serializer_json stays because the Promise reducer closes over per-request state.

No new test, the existing serialization-* suites in the basics app cover both paths.

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Install the latest version of @sveltejs/kit from ce4cf1d:

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