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/render.ts

@@ -0,0 +1,118 @@
+/**
+ * Plain-text transcript rendering over session events: the shared projection
+ * used wherever a compaction-class consumer needs "what a model once saw" as
+ * readable text — a summarizer's input, or a recall tool's output.
+ *
+ * Extracted from the basic backend's private helpers so the summarize path and
+ * the recall read path render one span identically (two renderers would drift,
+ * and a recall reader would then see a different transcript than the one the
+ * summary was written from). Both functions are pure over their arguments: no
+ * session access beyond the provided events, no clock, no randomness — a
+ * rendered span is a pure function of the log, so replay reproduces it
+ * byte-identically.
+ *
+ * @module @deepseek-ai/dsh-compact/render
+ */
+
+import type { ContentBlock } from '@deepseek-ai/dsh-llm'
+import type { SessionEvent } from '@deepseek-ai/dsh-session'
+
+/**
+ * Render content blocks to a single plain-text string. Text and reasoning
+ * contribute their text (reasoning wrapped as `[reasoning: …]`); every other
+ * block type contributes a type-tagged placeholder (`[tool-call: name(args)]`,
+ * `[tool-result: …]`, …) so the reader is told what non-text content existed
+ * rather than silently losing it. A `tool-result` block recurses into its
+ * nested content (`[tool-result: <inner rendering>]`), falling back to a bare
+ * `[tool-result]` when the nested content renders to nothing. Blocks join
+ * with newlines; empty-text blocks contribute nothing.
+ *
+ * @param blocks - the content blocks to render.
+ * @returns the newline-joined plain-text rendering; empty string when nothing renders.
+ */
+export function renderContentBlocks(blocks: readonly ContentBlock[]): string {
+  const parts: string[] = []
+  for (const block of blocks) {
+    switch (block.type) {
+      case 'text':
+        if (block.text) parts.push(block.text)
+        break
+      case 'reasoning':
+        if (block.text) parts.push(`[reasoning: ${block.text}]`)
+        break
+      case 'tool-call':
+        parts.push(`[tool-call: ${block.name}(${block.arguments})]`)
+        break
+      case 'tool-result': {
+        const inner = renderContentBlocks(block.content)
+        parts.push(inner ? `[tool-result: ${inner}]` : '[tool-result]')
+        break
+      }
+      // ContentBlockMap is merge-extensible — render an unknown block as a
+      // bare type-tagged placeholder so a plugin-added block type is still
+      // signalled to the reader rather than dropped.
+      default:
+        parts.push(`[${(block as ContentBlock).type}]`)
+    }
+  }
+  return parts.join('\n')
+}
+
+/**
+ * Render a set of surface-node seqs as a `User:`/`Assistant:`/`Tool result:`
+ * transcript. Walks `seqs` in the order given — callers pass surface order
+ * (e.g. a `compactRegion` slice of the surface-node list), which after a
+ * `replace` is NOT ascending log-seq order (a high-seq summary node can sit at
+ * the head of the surface before older retained lower-seq nodes); a log-order
+ * scan would render the transcript out of order.
+ *
+ * Only the five surface (message-producing) event types render; a seq naming
+ * any other event type contributes nothing. `SessionEventMap` is
+ * merge-extensible, so unknown types are simply non-message events with no
+ * renderable text.
+ *
+ * @param events - the session log the seqs index into (`session.events`).
+ * @param seqs - the surface-node seqs to render, in surface order.
+ * @returns the transcript, entries joined by blank lines; empty string when nothing renders.
+ */
+export function renderTranscript(events: readonly SessionEvent[], seqs: readonly number[]): string {
+  const lines: string[] = []
+
+  for (const seq of seqs) {
+    const event = events[seq]
+    if (!event) continue
+
+    switch (event.type) {
+      case 'user/message': {
+        const text = renderContentBlocks(event.data.content)
+        if (text) lines.push(`User: ${text}`)
+        break
+      }
+      case 'assistant/message': {
+        const text = renderContentBlocks(event.data.content)
+        if (text) lines.push(`Assistant: ${text}`)
+        break
+      }
+      case 'tool/result': {
+        const text = renderContentBlocks(event.data.content)
+        const label = event.data.isError ? 'Tool error' : 'Tool result'
+        if (text) lines.push(`${label} (call ${event.data.callId}): ${text}`)
+        break
+      }
+      case 'context/message': {
+        const text = renderContentBlocks(event.data.content)
+        if (text) lines.push(`[Context: ${text}]`)
+        break
+      }
+      case 'steering/message': {
+        const text = renderContentBlocks(event.data.content)
+        if (text) lines.push(`[Steering: ${text}]`)
+        break
+      }
+      default:
+        break
+    }
+  }
+
+  return lines.join('\n\n')
+}