feat(tui): honor display.* flags in turn renderer, status bar, and event handler

- turnController gates scheduleStreaming / reasoning recorders on
  streaming + showReasoning so disabling them keeps the buffer silent
  until message.complete flushes
- createGatewayEventHandler only surfaces inline_diff previews when
  inlineDiffs is on
- StatusRule takes a showCost prop and renders `· $X.XXXX` with the
  same toFixed(4) formatting as /usage when usage.cost_usd is present
- Usage grows cost_usd?: number to match the gateway payload
- Existing handler tests flip showReasoning on in beforeEach so
  reasoning-flow assertions keep their meaning

ui-tui/src/components/appChrome.tsx

@@ -99,6 +99,7 @@ export function StatusRule({
   usage,
   bgCount,
   sessionStartedAt,
+  showCost,
   voiceLabel,
   t
 }: StatusRuleProps) {
@@ -136,6 +137,9 @@ export function StatusRule({
           ) : null}
           {voiceLabel ? <Text color={t.color.dim}> │ {voiceLabel}</Text> : null}
           {bgCount > 0 ? <Text color={t.color.dim}> │ {bgCount} bg</Text> : null}
+          {showCost && typeof usage.cost_usd === 'number' ? (
+            <Text color={t.color.dim}> │ ${usage.cost_usd.toFixed(4)}</Text>
+          ) : null}
         </Text>
       </Box>
 
@@ -285,6 +289,7 @@ interface StatusRuleProps {
   cwdLabel: string
   model: string
   sessionStartedAt?: number | null
+  showCost: boolean
   status: string
   statusColor: string
   t: Theme

ui-tui/src/components/appLayout.tsx

@@ -190,6 +190,7 @@ const ComposerPane = memo(function ComposerPane({
             cwdLabel={status.cwdLabel}
             model={ui.info?.model?.split('/').pop() ?? ''}
             sessionStartedAt={status.sessionStartedAt}
+            showCost={ui.showCost}
             status={ui.status}
             statusColor={status.statusColor}
             t={ui.theme}