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-macos/Package.swift

@@ -0,0 +1,37 @@
+// swift-tools-version: 5.9
+
+import PackageDescription
+
+// Path to the Rust static library built by `cargo build --release`
+let rustLibPath = "../calcpad-engine/target/release"
+
+let package = Package(
+    name: "CalcPad",
+    platforms: [
+        .macOS(.v14)
+    ],
+    targets: [
+        .systemLibrary(
+            name: "CCalcPadEngine",
+            path: "Sources/CCalcPadEngine"
+        ),
+        .executableTarget(
+            name: "CalcPad",
+            dependencies: ["CCalcPadEngine"],
+            path: "Sources/CalcPad",
+            linkerSettings: [
+                .unsafeFlags(["-L\(rustLibPath)"]),
+                .linkedLibrary("calcpad_engine"),
+            ]
+        ),
+        .testTarget(
+            name: "CalcPadTests",
+            dependencies: ["CalcPad"],
+            path: "Tests/CalcPadTests",
+            linkerSettings: [
+                .unsafeFlags(["-L\(rustLibPath)"]),
+                .linkedLibrary("calcpad_engine"),
+            ]
+        ),
+    ]
+)