Use explicit ts specifiers for declarations

Restore explicit .ts relative specifiers in source and enable rewriteRelativeImportExtensions so emitted JS uses .js while declarations keep explicit .ts specifiers. Add a NodeNext declaration-consumer gate to prevent extensionless declaration regressions.

packages/ui/acp/src/index.ts

@@ -74,7 +74,7 @@ import {
   harnessBlockToAcpContent,
   promptHasUnsupportedContent,
   turnEndToStopReason,
-} from './codec'
+} from './codec.ts'
 
 export const name = 'acp'
 // The bridge programs against the interface packages only (architecture rule: