fix(tui): reduce status bar flicker during streaming

The status bar flickers visibly during streaming because every state
patch (thinking.delta, reasoning.delta, tool.*, usage notifications)
creates a new $uiState object, forcing StatusRulePane and StatusRule
to re-render and redo expensive layout calculations on every event.

Three fixes applied:

1. Stabilize usage object references in createGatewayEventHandler.ts
   - Add mergeUsageStable() that shallow-compares Usage fields before
     creating a new object. When values haven't changed, returns the
     existing reference, preventing unnecessary StatusRule re-renders.

2. Memoize expensive computations inside StatusRule (appChrome.tsx)
   - statusBarSegments(cols) → useMemo([cols])
   - modelLabel() → useMemo([model, effort, fast])
   - ctxLabel, bar → useMemo([usage fields, segs])
   - Tail segment budget + fits() calculations → single useMemo block
     covering all progressive-disclosure logic

3. Wrap StatusRule in React.memo (appChrome.tsx)
   - Combined with stable usage references, allows React to skip
     re-renders when props haven't actually changed.

Fixes #41480

ui-tui/src/app/createGatewayEventHandler.ts

@@ -23,7 +23,7 @@ import { isPaintableHex, setTerminalBackground, setTerminalForeground } from '..
 import { formatAbandonedClarify, formatToolCall, stripAnsi } from '../lib/text.js'
 import { bootSeededPin, invalidateBootBackground, writeBootTheme } from '../lib/themeBoot.js'
 import { defaultThemeForCurrentBackground, fromSkin, skinIsLight, type Theme, themeToneHex } from '../theme.js'
-import type { Msg, SubagentProgress, SubagentStatus } from '../types.js'
+import type { Msg, SubagentProgress, SubagentStatus, Usage } from '../types.js'
 
 import { applyDelegationStatus, getDelegationState } from './delegationStore.js'
 import type { GatewayEventHandlerContext } from './interfaces.js'
@@ -41,6 +41,28 @@ type VoiceSubmitMode = 'direct' | 'draft'
 const normalizeVoiceSubmitMode = (value: unknown): VoiceSubmitMode =>
   typeof value === 'string' && value.trim().toLowerCase() === 'draft' ? 'draft' : 'direct'
 
+// Shallow-compare Usage to avoid creating a new object reference when values
+// haven't changed. A fresh reference on every streaming event forces every
+// $uiState subscriber (including the status rule) to re-render, which showed
+// up as per-delta status-bar flicker on iTerm2 (#41480). The comparator
+// iterates the union of keys generically so a future Usage field (e.g.
+// active_subagents, consumed by the status rule's subagent segment) can never
+// be silently dropped from the comparison.
+const usageChanged = (prev: Usage, next: Usage): boolean => {
+  const keys = new Set([...Object.keys(prev), ...Object.keys(next)]) as Set<keyof Usage>
+  for (const key of keys) {
+    if (prev[key] !== next[key]) return true
+  }
+  return false
+}
+
+const mergeUsageStable = (prev: Usage, patch: Partial<Usage> | undefined): Usage => {
+  if (!patch) return prev
+  const merged: Usage = { ...prev, ...patch }
+  return usageChanged(prev, merged) ? merged : prev
+}
+
+
 const statusFromBusy = () => (getUiState().busy ? 'running…' : 'ready')
 
 // The last gateway skin, kept so the theme can be re-derived when the OSC-11
@@ -745,7 +767,7 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
           ...state,
           info,
           status: state.status === 'starting agent…' ? 'ready' : state.status,
-          usage: info.usage ? { ...state.usage, ...info.usage } : state.usage
+          usage: info.usage ? mergeUsageStable(state.usage, info.usage) : state.usage
         }))
 
         setHistoryItems(prev => prev.map(m => (m.kind === 'intro' ? { ...m, info } : m)))
@@ -1370,7 +1392,7 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
         setStatus('ready')
 
         if (ev.payload?.usage) {
-          patchUiState(state => ({ ...state, usage: { ...state.usage, ...ev.payload!.usage } }))
+          patchUiState(state => ({ ...state, usage: mergeUsageStable(state.usage, ev.payload!.usage) }))
         }
 
         // Billing wall (out of credits / payment required): open a proper