docs(compact): correct pruning surface contracts

packages/compact/compact-tool-result-prune/README.md

@@ -8,6 +8,8 @@ This is a concrete companion to [`dsh-compact-basic`](../compact-basic/README.md
 
 `pruneSession(session)` scans one stable snapshot of the current surface. Every over-budget tool result is replaced by one newly appended `tool/result` carrying `{ surfaceOp: { op: 'replace', start: originalSeq, end: originalSeq }, sourceEventSeqs: [originalSeq] }`. The replacement spreads the complete original data and changes only `content`, preserving `turn`, `step`, `callId`, error fields, `meta`, and later data additions. The original event remains available for persistence, replay, and exact-log inspection.
 
+The method throws synchronously when the session rejects a replacement. Replacements committed earlier in the pass remain durable.
+
 `measureContent(blocks)` counts Unicode code points in `text` blocks. `pruneContent(blocks)` returns the bounded replacement or `null` when content is already within the threshold. Non-text blocks are retained at their original relative positions; text slicing never splits a UTF-16 surrogate pair, though it can split a multi-code-point grapheme cluster.
 
 Every emitted result has exactly the configured head budget, fixed marker, and tail budget in text code points, is no larger than `thresholdChars`, and is strictly smaller than the triggering input. A second pass therefore emits no replacement.

packages/compact/compact-tool-result-prune/src/index.ts

@@ -118,6 +118,8 @@ export class ToolResultPruneService extends Service {
    * and points at the shadowed node for durable provenance and replay.
    * @param session - session whose current surface is rewritten.
    * @returns landed replacements and aggregate Unicode-code-point savings.
+   * @throws when the session rejects a replacement; replacements committed
+   * earlier in the pass remain durable.
    */
   pruneSession(session: Session): PruneResult {
     const candidates: SnapshotCandidate[] = []