refactor(compact): extract the shared transcript renderer into dsh-compact

Move compact-basic's private _extractText/_blocksToText into the interface
package as renderTranscript/renderContentBlocks, so the summarize path and a
future recall read path render one span identically. Byte-identical output
vs the private helpers it replaces; compact-basic delegates.

packages/compact/compact/src/index.ts

@@ -26,6 +26,7 @@ import type { Session } from '@deepseek-ai/dsh-session'
 import type { CompactionResult } from './types.ts'
 
 export type { CompactionResult } from './types.ts'
+export { renderContentBlocks, renderTranscript } from './render.ts'
 
 /** Minimal agent context compaction needs without depending on the agent package. */
 export interface CompactAgentContext {