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/Sources/CalcPad/Views/TwoColumnEditorView.swift

@@ -0,0 +1,72 @@
+import Combine
+import SwiftUI
+
+/// The main two-column editor layout: text editor on the left, results on the right.
+/// Scrolling is synchronized between both columns.
+struct TwoColumnEditorView: View {
+    @State private var text: String = ""
+    @State private var scrollOffset: CGFloat = 0
+    @State private var results: [LineResult] = []
+    @State private var evaluationTask: Task<Void, Never>?
+
+    /// Uses the Rust FFI engine. Falls back to StubCalculationEngine if the Rust
+    /// library is not linked (e.g., during UI-only development).
+    private let engine: CalculationEngine = RustCalculationEngine()
+
+    /// Debounce interval for re-evaluation after typing (seconds).
+    private let evaluationDebounce: TimeInterval = 0.05
+
+    /// Font that respects the user's accessibility / Dynamic Type settings.
+    private var editorFont: NSFont {
+        // Use the system's preferred monospaced font size, which scales with
+        // Accessibility > Display > Text Size in System Settings (macOS 14+).
+        let baseSize = NSFont.systemFontSize
+        // Scale with accessibility settings via the body text style size
+        let preferredSize = NSFont.preferredFont(forTextStyle: .body, options: [:]).pointSize
+        // Use the larger of system default or accessibility-preferred size
+        let size = max(baseSize, preferredSize)
+        return NSFont.monospacedSystemFont(ofSize: size, weight: .regular)
+    }
+
+    var body: some View {
+        HSplitView {
+            // Left pane: Editor
+            EditorTextView(
+                text: $text,
+                scrollOffset: $scrollOffset,
+                font: editorFont
+            )
+            .frame(minWidth: 200)
+
+            // Divider is automatic with HSplitView
+
+            // Right pane: Answer column
+            AnswerColumnView(
+                results: results,
+                scrollOffset: $scrollOffset,
+                font: editorFont
+            )
+            .frame(minWidth: 120, idealWidth: 200)
+        }
+        .onChange(of: text) { _, newValue in
+            scheduleEvaluation(newValue)
+        }
+        .onAppear {
+            evaluateText(text)
+        }
+    }
+
+    /// Debounce evaluation so rapid typing doesn't cause excessive recalculation.
+    private func scheduleEvaluation(_ newText: String) {
+        evaluationTask?.cancel()
+        evaluationTask = Task { @MainActor in
+            try? await Task.sleep(for: .milliseconds(Int(evaluationDebounce * 1000)))
+            guard !Task.isCancelled else { return }
+            evaluateText(newText)
+        }
+    }
+
+    private func evaluateText(_ newText: String) {
+        results = engine.evaluateSheet(newText)
+    }
+}