Usage popover: label rows as session totals

Section header 'Session totals' + pluralized cache rows make the
cumulative semantics explicit.

surfaces/gui/e2e/usage-chip.spec.ts

@@ -30,9 +30,11 @@ test("usage chip appears after a turn and opens the breakdown popover", async ({
   await expect(pop).toBeVisible();
   await expect(pop).toContainText("Context window");
   await expect(pop).toContainText("9.8k of 200k · 5%");
+  await expect(pop).toContainText("Session totals");
   await expect(pop).toContainText("Claude Opus 4.8 · Anthropic");
   await expect(pop).toContainText("Input");
-  await expect(pop).toContainText("Cache read");
+  await expect(pop).toContainText("Cache reads");
+  await expect(pop).toContainText("Cache writes");
   // Total input = fresh 1k + cache_read 8k + cache_write 800 (cumulative billed input).
   await expect(pop).toContainText("Total input");
   await expect(pop).toContainText("9.8k");

surfaces/gui/src/components/Composer.tsx

@@ -650,7 +650,7 @@ function UsageChip({
               </div>
             ) : null}
             <div className="text-[10.5px] uppercase tracking-[0.06em] text-faint font-semibold mb-1">
-              This session
+              Session totals
             </div>
             <div className="flex flex-col gap-1.5">
               {Object.entries(usage.byModel).map(([id, t]) => (
@@ -660,8 +660,8 @@ function UsageChip({
                   </div>
                   <div className="mt-0.5 flex flex-col gap-0.5">
                     {stat("Input", t.input)}
-                    {t.cache_read > 0 && stat("Cache read", t.cache_read)}
-                    {t.cache_write > 0 && stat("Cache write", t.cache_write)}
+                    {t.cache_read > 0 && stat("Cache reads", t.cache_read)}
+                    {t.cache_write > 0 && stat("Cache writes", t.cache_write)}
                     {/* Cumulative billed input volume = fresh + cache reads + cache
                         writes (each class bills at its own rate). Shown only when a
                         cache split exists — otherwise it would duplicate Input. */}