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/vite.config.ts

@@ -0,0 +1,95 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+import { VitePWA } from 'vite-plugin-pwa'
+
+export default defineConfig({
+  plugins: [
+    react(),
+    VitePWA({
+      registerType: 'prompt',
+      includeAssets: ['favicon.svg', 'icons/*.svg'],
+      manifest: {
+        name: 'CalcPad',
+        short_name: 'CalcPad',
+        description: 'A modern notepad calculator powered by WebAssembly',
+        theme_color: '#6366f1',
+        background_color: '#ffffff',
+        display: 'standalone',
+        orientation: 'any',
+        scope: '/',
+        start_url: '/',
+        icons: [
+          {
+            src: '/icons/icon-192.svg',
+            sizes: '192x192',
+            type: 'image/svg+xml',
+            purpose: 'any',
+          },
+          {
+            src: '/icons/icon-512.svg',
+            sizes: '512x512',
+            type: 'image/svg+xml',
+            purpose: 'any',
+          },
+          {
+            src: '/icons/icon-maskable-512.svg',
+            sizes: '512x512',
+            type: 'image/svg+xml',
+            purpose: 'maskable',
+          },
+        ],
+        categories: ['productivity', 'utilities'],
+      },
+      workbox: {
+        globPatterns: ['**/*.{js,css,html,svg,wasm}'],
+        runtimeCaching: [
+          {
+            urlPattern: /^https:\/\/api\./,
+            handler: 'NetworkFirst',
+            options: {
+              cacheName: 'api-cache',
+              expiration: {
+                maxEntries: 50,
+                maxAgeSeconds: 60 * 60 * 24,
+              },
+              networkTimeoutSeconds: 5,
+            },
+          },
+          {
+            urlPattern: /\.(?:png|jpg|jpeg|svg|gif|webp)$/,
+            handler: 'CacheFirst',
+            options: {
+              cacheName: 'image-cache',
+              expiration: {
+                maxEntries: 100,
+                maxAgeSeconds: 60 * 60 * 24 * 30,
+              },
+            },
+          },
+        ],
+      },
+      devOptions: {
+        enabled: false,
+      },
+    }),
+  ],
+  build: {
+    target: 'es2022',
+    rollupOptions: {
+      output: {
+        manualChunks: {
+          react: ['react', 'react-dom'],
+          codemirror: [
+            '@codemirror/state',
+            '@codemirror/view',
+            '@codemirror/language',
+            '@lezer/highlight',
+          ],
+        },
+      },
+    },
+  },
+  worker: {
+    format: 'es',
+  },
+})