revert(tui): drop DeferredMd, profiling showed it was neutral

Profiled with scripts/profile-tui.py under hold-PageUp + hold-wheel.
The placeholder → microtask-upgrade pattern did not reduce renderer
p99 (63ms → 63ms) or max (96ms → 142ms, slightly worse).  Each fresh
row still pays the Md cost — just on a follow-up commit instead of
inline — and the follow-up commit shows up as a second heavy frame
a few ms later.

The real bottlenecks turned out to be:

  1. wheel step too large (fixed in 7ca16eea)
  2. outer terminal ANSI parse throughput (diagnosing next)
  3. React commit frequency during hold-scroll (needs coalescing)

None of which DeferredMd addresses.  Clearing the complexity so the
next experiments land on a simpler substrate.

ui-tui/src/components/thinking.tsx

@@ -899,7 +899,7 @@ export const ToolTrail = memo(function ToolTrail({
     return duration ? (
       <>
         {label}
-        <Text color={t.color.dim} dim>
+        <Text color={t.color.statusFg} dim>
           {duration}
         </Text>
       </>