fix(turn-metrics): exact per-turn placement via turnId, decode-only TPS, step-level retention

Placement
- Chunks carry their producing turn: StoredChunk.turnId (backend [email protected]) for
  committed rows (persisted in the IDB cache; old cached rows are upgraded on
  sync) and the live event's turnId for provisional ones (the optimistic echo is
  tagged on turn-start).
- interleaveTurnMetrics attributes every group to its turn (turnId, else the
  turn owning its stepId) and puts the turn total after the turn's LAST visible
  group. No user-message counting: sending, streaming, steering, images, errored
  turns and chat-limit unloading can no longer shift metrics between turns — at
  any point during a turn, not just after it ends. A turn cut in half by the
  chat limit keeps its total under its visible tail.
- Legacy rows (no turnId) fall back to an order-preserving, count-guarded match
  that leaves a turn blank rather than show another turn's numbers.
- A turn that errors re-syncs metrics (the backend now persists them).

TPS
- Step TPS = outputTokens / decodeMs only (no genTotalMs fallback — it includes
  time-to-first-token). Turn TPS sums tokens and decode time over timed steps
  only. Tool execution is excluded by the backend.

Display
- Retention = this turn's first-step cache reads / previous turn's last-step
  prompt (step-level; turn sums made it pin at 100%).
- Turn row shows output tokens + input processed (with cached), instead of a
  misleading 'total tok' that summed every step's full prompt.

1 files changed, 21 insertions, 7 deletions

diff --git a/src/core/chunks/selectors.ts b/src/core/chunks/selectors.ts
index e46d5f5..8b265a5 100644
--- a/src/core/chunks/selectors.ts
+++ b/src/core/chunks/selectors.ts
@@ -8,17 +8,31 @@ import type { RenderedChunk, TranscriptState } from "./types";
 export function selectChunks(state: TranscriptState): readonly RenderedChunk[] {
   const result: RenderedChunk[] = [];
   for (const c of state.committed) {
-    result.push({ seq: c.seq, role: c.role, chunk: c.chunk, provisional: false });
+    result.push(
+      c.turnId !== undefined
+        ? { seq: c.seq, role: c.role, chunk: c.chunk, provisional: false, turnId: c.turnId }
+        : { seq: c.seq, role: c.role, chunk: c.chunk, provisional: false },
+    );
   }
   for (const p of state.provisional) {
-    result.push({ seq: null, role: p.role, chunk: p.chunk, provisional: true });
+    result.push(
+      p.turnId !== undefined
+        ? { seq: null, role: p.role, chunk: p.chunk, provisional: true, turnId: p.turnId }
+        : { seq: null, role: p.role, chunk: p.chunk, provisional: true },
+    );
   }
-  if (state.accumulating !== null) {
+  const acc = state.accumulating;
+  if (acc !== null) {
     const chunk: Chunk =
-      state.accumulating.kind === "text"
-        ? { type: "text", text: state.accumulating.text }
-        : { type: "thinking", text: state.accumulating.text };
-    result.push({ seq: null, role: "assistant", chunk, provisional: true, streaming: true });
+      acc.kind === "text" ? { type: "text", text: acc.text } : { type: "thinking", text: acc.text };
+    result.push({
+      seq: null,
+      role: "assistant",
+      chunk,
+      provisional: true,
+      streaming: true,
+      ...(acc.turnId !== undefined ? { turnId: acc.turnId } : {}),
+    });
   }
   return result;
 }