refactor(runtime): collapse speculative portability layers

Remove the one-consumer bounded-read primitive and shared terminal lifecycle controller, make terminal cleanup one awaited provider operation, and reuse one Code Runtime contract suite. Keep only reproduced cancellation and policy fixes; defer unproven replacement, prompt-attribution, and streaming-frame concerns to scoped markers.

packages/code-runtime/code-runtime-worker/src/runtime-host.ts

@@ -223,5 +223,4 @@ export class RuntimeOutputLedger {
 }
 
 export { decodeWorkerJson, encodeWorkerJson, snapshotCodeJsonValue } from './worker-json.ts'
-export { jsonValueBytesUpTo } from './output-json.ts'
 export type { WorkerJsonWire } from './worker-json.ts'