compact: the summary's provenance records its call envelope

compact/summary gains { model, maxTokens? } — the envelope the
summarize call actually used, reported by the backend that made the
call: summarize() now returns { summary, model, maxTokens? } instead of
bare blocks, so an overriding backend (template or remote summarizer)
reports its own envelope honestly and compactRegion logs it. 'Which
model wrote this summary' becomes answerable from the log alone, and
the one-shot summarize request — outside the loop's header-event fold
by design — is reconstructable from log + code (the reconstructability
RFC's scope statement).

packages/compact/compact-basic/tests/compact-loop-repro.spec.ts

@@ -41,8 +41,8 @@ class ReproCompactService extends BasicCompactService {
     return blocks.length * TOKENS_PER_BLOCK
   }
 
-  override async summarize(): Promise<ContentBlock[]> {
-    return [{ type: 'text', text: 'CHECKPOINT SUMMARY' }]
+  override async summarize(): Promise<{ summary: ContentBlock[]; model: string }> {
+    return { summary: [{ type: 'text', text: 'CHECKPOINT SUMMARY' }], model: 'stub' }
   }
 }