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/session-persistence/session-persistence/src/coordinator.ts

@@ -27,7 +27,7 @@
 import { Context } from 'cordis'
 import { interruptedTurnClosers, SESSION_FORMAT_VERSION } from '@deepseek-ai/dsh-session'
 import type { Session, SessionEvent, SessionId, SessionHeader } from '@deepseek-ai/dsh-session'
-import { assertSerializable, seedCoversPrefix } from './index'
+import { assertSerializable, seedCoversPrefix } from './index.ts'
 
 /**
  * A stored session's durable prefix as read back from a backend: its

packages/session-persistence/session-persistence/src/index.ts

@@ -29,8 +29,8 @@ import type { SessionEvent, SessionId, SessionHeader } from '@deepseek-ai/dsh-se
 export type { SessionHeader } from '@deepseek-ai/dsh-session'
 
 // The backend-agnostic write-path orchestration first-party backends compose.
-export { PersistenceCoordinator } from './coordinator'
-export type { PersistenceBackend, StoredPrefix } from './coordinator'
+export { PersistenceCoordinator } from './coordinator.ts'
+export type { PersistenceBackend, StoredPrefix } from './coordinator.ts'
 
 declare module 'cordis' {
   interface Context {