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/App.tsx

@@ -0,0 +1,89 @@
+/**
+ * CalcPad main application component.
+ *
+ * Two-column layout:
+ *   Left:  CodeMirror 6 editor with CalcPad syntax highlighting
+ *   Right: Answer gutter (integrated into CodeMirror) + optional standalone AnswerColumn
+ *
+ * The WASM engine runs in a Web Worker. On each document change (debounced),
+ * the editor sends lines to the worker, which evaluates them and posts back
+ * results. Results are fed into the CodeMirror answer gutter extension.
+ */
+
+import { useCallback } from 'react'
+import { CalcEditor } from './editor/CalcEditor.tsx'
+import { useEngine } from './engine/useEngine.ts'
+import { useOnlineStatus } from './hooks/useOnlineStatus.ts'
+import { useInstallPrompt } from './hooks/useInstallPrompt.ts'
+import { OfflineBanner } from './components/OfflineBanner.tsx'
+import { InstallPrompt } from './components/InstallPrompt.tsx'
+import './styles/app.css'
+
+const INITIAL_DOC = `# CalcPad
+
+// Basic arithmetic
+2 + 3
+10 * 4.5
+100 / 7
+
+// Variables
+price = 49.99
+quantity = 3
+subtotal = price * quantity
+
+// Percentages
+tax = subtotal * 8%
+total = subtotal + tax
+
+// Functions
+sqrt(144)
+2 ^ 10
+`
+
+function App() {
+  const engine = useEngine()
+  const isOnline = useOnlineStatus()
+  const installPrompt = useInstallPrompt()
+
+  const handleDocChange = useCallback(
+    (lines: string[]) => {
+      engine.evalSheet(lines)
+    },
+    [engine.evalSheet],
+  )
+
+  return (
+    <div className="calcpad-app">
+      <OfflineBanner isOnline={isOnline} />
+
+      <header className="calcpad-header">
+        <h1>CalcPad</h1>
+        <p className="subtitle">Notepad Calculator</p>
+        <div className="header-status">
+          <span className={`status-dot ${engine.ready ? '' : 'loading'}`} />
+          <span>{engine.ready ? 'Engine ready' : 'Loading engine...'}</span>
+        </div>
+      </header>
+
+      <main className="calcpad-editor">
+        <div className="editor-pane">
+          <CalcEditor
+            initialDoc={INITIAL_DOC}
+            onDocChange={handleDocChange}
+            results={engine.results}
+            debounceMs={50}
+          />
+        </div>
+      </main>
+
+      <InstallPrompt
+        promptEvent={installPrompt.promptEvent}
+        isInstalled={installPrompt.isInstalled}
+        onInstall={installPrompt.handleInstall}
+        onDismiss={installPrompt.handleDismiss}
+      />
+    </div>
+  )
+}
+
+export default App