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/EditorTextView.swift

@@ -0,0 +1,137 @@
+import AppKit
+import SwiftUI
+
+/// A SwiftUI wrapper around NSTextView for the editor pane.
+/// Uses NSViewRepresentable to bridge AppKit's NSTextView into SwiftUI,
+/// providing line-level control, scroll position access, and performance
+/// that SwiftUI's TextEditor cannot match.
+struct EditorTextView: NSViewRepresentable {
+    @Binding var text: String
+    @Binding var scrollOffset: CGFloat
+    var font: NSFont
+
+    func makeCoordinator() -> Coordinator {
+        Coordinator(self)
+    }
+
+    func makeNSView(context: Context) -> NSScrollView {
+        let scrollView = NSScrollView()
+        scrollView.hasVerticalScroller = true
+        scrollView.hasHorizontalScroller = false
+        scrollView.autohidesScrollers = true
+        scrollView.borderType = .noBorder
+        scrollView.drawsBackground = false
+
+        let textView = NSTextView()
+        textView.isEditable = true
+        textView.isSelectable = true
+        textView.allowsUndo = true
+        textView.isRichText = false
+        textView.usesFontPanel = false
+        textView.isAutomaticQuoteSubstitutionEnabled = false
+        textView.isAutomaticDashSubstitutionEnabled = false
+        textView.isAutomaticTextReplacementEnabled = false
+        textView.isAutomaticSpellingCorrectionEnabled = false
+
+        // Disable line wrapping — horizontal scroll instead
+        textView.isHorizontallyResizable = true
+        textView.textContainer?.widthTracksTextView = false
+        textView.textContainer?.containerSize = NSSize(
+            width: CGFloat.greatestFiniteMagnitude,
+            height: CGFloat.greatestFiniteMagnitude
+        )
+        textView.maxSize = NSSize(
+            width: CGFloat.greatestFiniteMagnitude,
+            height: CGFloat.greatestFiniteMagnitude
+        )
+
+        // Configure font and text color
+        textView.font = font
+        textView.textColor = .textColor
+        textView.backgroundColor = .clear
+        textView.drawsBackground = false
+        textView.insertionPointColor = .textColor
+
+        // Set the text
+        textView.string = text
+
+        // Configure the text container for consistent line spacing
+        textView.textContainer?.lineFragmentPadding = 4
+        textView.textContainerInset = NSSize(width: 8, height: 8)
+
+        scrollView.documentView = textView
+        context.coordinator.textView = textView
+        context.coordinator.scrollView = scrollView
+
+        // Observe text changes
+        textView.delegate = context.coordinator
+
+        // Observe scroll changes
+        scrollView.contentView.postsBoundsChangedNotifications = true
+        NotificationCenter.default.addObserver(
+            context.coordinator,
+            selector: #selector(Coordinator.scrollViewDidScroll(_:)),
+            name: NSView.boundsDidChangeNotification,
+            object: scrollView.contentView
+        )
+
+        return scrollView
+    }
+
+    func updateNSView(_ scrollView: NSScrollView, context: Context) {
+        guard let textView = scrollView.documentView as? NSTextView else { return }
+
+        // Update font if changed
+        if textView.font != font {
+            textView.font = font
+            // Reapply font to entire text storage
+            let range = NSRange(location: 0, length: textView.textStorage?.length ?? 0)
+            textView.textStorage?.addAttribute(.font, value: font, range: range)
+        }
+
+        // Update text only if it actually changed (avoid feedback loops)
+        if textView.string != text {
+            let selectedRanges = textView.selectedRanges
+            textView.string = text
+            textView.selectedRanges = selectedRanges
+        }
+
+        // Update scroll position if driven externally
+        context.coordinator.isUpdatingScroll = true
+        let currentOffset = scrollView.contentView.bounds.origin.y
+        if abs(currentOffset - scrollOffset) > 0.5 {
+            scrollView.contentView.scroll(to: NSPoint(x: 0, y: scrollOffset))
+            scrollView.reflectScrolledClipView(scrollView.contentView)
+        }
+        context.coordinator.isUpdatingScroll = false
+    }
+
+    final class Coordinator: NSObject, NSTextViewDelegate {
+        var parent: EditorTextView
+        weak var textView: NSTextView?
+        weak var scrollView: NSScrollView?
+        var isUpdatingScroll = false
+
+        init(_ parent: EditorTextView) {
+            self.parent = parent
+        }
+
+        func textDidChange(_ notification: Notification) {
+            guard let textView = notification.object as? NSTextView else { return }
+            parent.text = textView.string
+        }
+
+        @objc func scrollViewDidScroll(_ notification: Notification) {
+            guard !isUpdatingScroll,
+                  let scrollView = scrollView else { return }
+            let offset = scrollView.contentView.bounds.origin.y
+            DispatchQueue.main.async { [weak self] in
+                self?.parent.scrollOffset = offset
+            }
+        }
+
+        deinit {
+            NotificationCenter.default.removeObserver(self)
+        }
+    }
+}