feat: add platform shells, CLI, formatting, plugins, tests, and benchmarks

Phase 4 — Platform shells:
- calcpad-macos/: SwiftUI two-column editor with Rust FFI bridge (16 files)
- calcpad-windows/: iced GUI with Windows 11 Fluent theme (7 files, 13 tests)
- calcpad-web/: React 18 + CodeMirror 6 + WASM Worker + PWA (20 files)
- calcpad-cli/: clap-based CLI with expression eval, pipe/stdin, JSON/CSV
  output, and interactive REPL with rustyline history

Phase 5 — Engine modules:
- formatting/: answer formatting (decimal/scientific/SI notation, thousands
  separators, currency), line type classification, clipboard values (93 tests)
- plugins/: CalcPadPlugin trait, PluginRegistry, Rhai scripting stub (43 tests)
- benches/: criterion benchmarks (single-line, 100/500-line sheets, DAG, incremental)
- tests/sheet_scenarios.rs: 20 real-world integration tests
- tests/proptest_fuzz.rs: 12 property-based fuzz tests

771 tests passing across workspace, 0 failures.

calcpad-web/src/hooks/useInstallPrompt.ts

@@ -0,0 +1,69 @@
+/**
+ * React hook that captures the beforeinstallprompt event for PWA install.
+ * Adapted from epic/9-3-pwa-support.
+ */
+
+import { useState, useEffect, useCallback } from 'react'
+
+interface BeforeInstallPromptEvent extends Event {
+  prompt(): Promise<void>
+  userChoice: Promise<{ outcome: 'accepted' | 'dismissed' }>
+}
+
+export interface InstallPromptState {
+  promptEvent: BeforeInstallPromptEvent | null
+  isInstalled: boolean
+  handleInstall: () => Promise<void>
+  handleDismiss: () => void
+}
+
+export function useInstallPrompt(): InstallPromptState {
+  const [promptEvent, setPromptEvent] = useState<BeforeInstallPromptEvent | null>(null)
+  const [isInstalled, setIsInstalled] = useState(false)
+
+  useEffect(() => {
+    // Check if already installed (standalone mode)
+    const isStandalone =
+      window.matchMedia('(display-mode: standalone)').matches ||
+      ('standalone' in navigator && (navigator as unknown as { standalone: boolean }).standalone)
+
+    if (isStandalone) {
+      setIsInstalled(true)
+      return
+    }
+
+    const handler = (e: Event) => {
+      e.preventDefault()
+      setPromptEvent(e as BeforeInstallPromptEvent)
+    }
+
+    const installedHandler = () => {
+      setIsInstalled(true)
+      setPromptEvent(null)
+    }
+
+    window.addEventListener('beforeinstallprompt', handler)
+    window.addEventListener('appinstalled', installedHandler)
+
+    return () => {
+      window.removeEventListener('beforeinstallprompt', handler)
+      window.removeEventListener('appinstalled', installedHandler)
+    }
+  }, [])
+
+  const handleInstall = useCallback(async () => {
+    if (!promptEvent) return
+    await promptEvent.prompt()
+    const result = await promptEvent.userChoice
+    if (result.outcome === 'accepted') {
+      setIsInstalled(true)
+    }
+    setPromptEvent(null)
+  }, [promptEvent])
+
+  const handleDismiss = useCallback(() => {
+    setPromptEvent(null)
+  }, [])
+
+  return { promptEvent, isInstalled, handleInstall, handleDismiss }
+}

calcpad-web/src/hooks/useOnlineStatus.ts

@@ -0,0 +1,27 @@
+/**
+ * React hook that tracks whether the browser is online or offline.
+ * Adapted from epic/9-3-pwa-support.
+ */
+
+import { useState, useEffect } from 'react'
+
+export function useOnlineStatus(): boolean {
+  const [isOnline, setIsOnline] = useState(
+    typeof navigator !== 'undefined' ? navigator.onLine : true,
+  )
+
+  useEffect(() => {
+    const handleOnline = () => setIsOnline(true)
+    const handleOffline = () => setIsOnline(false)
+
+    window.addEventListener('online', handleOnline)
+    window.addEventListener('offline', handleOffline)
+
+    return () => {
+      window.removeEventListener('online', handleOnline)
+      window.removeEventListener('offline', handleOffline)
+    }
+  }, [])
+
+  return isOnline
+}