feat: trim chunks during generation via step-complete syncTail (CR-6)

The backend now persists chunks at step boundaries during generation
(CR-6). The FE calls syncTail on each step-complete event to fetch
the newly committed chunks. applyHistory clears the provisional array
when new committed chunks arrive mid-generation (they're duplicates
of what was folded from live events). The accumulating chunk (current
in-progress step) is kept.

This means trimTranscript can now drop oldest committed chunks
uniformly during a long turn — no unbounded provisional growth. The
browser never holds more than chatLimit chunks, even mid-generation.

3 new tests: clears provisional on new committed during generation,
keeps provisional when no new chunks, keeps accumulating when clearing.

689 tests green.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/features/chat/store.svelte.ts b/src/features/chat/store.svelte.ts
index 9beabfc..f6c23fc 100644
--- a/src/features/chat/store.svelte.ts
+++ b/src/features/chat/store.svelte.ts
@@ -284,6 +284,11 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
 			if (transcript.sealedTurnId !== null) {
 				void syncTail();
 				void syncMetrics();
+			} else if (msg.event.type === "step-complete") {
+				// CR-6: backend persists chunks at step boundaries. Fetch them
+				// as committed so trimTranscript can unload oldest chunks
+				// uniformly — no unbounded provisional growth during long turns.
+				void syncTail();
 			}
 		},