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/AnswerColumnView.swift

@@ -0,0 +1,111 @@
+import AppKit
+import SwiftUI
+
+/// Displays calculation results in a vertical column, one result per line,
+/// aligned to match the corresponding editor lines.
+/// Uses NSViewRepresentable wrapping NSScrollView + NSTextView for pixel-perfect
+/// line height alignment with the editor.
+struct AnswerColumnView: NSViewRepresentable {
+    let results: [LineResult]
+    @Binding var scrollOffset: CGFloat
+    var font: NSFont
+
+    func makeCoordinator() -> Coordinator {
+        Coordinator()
+    }
+
+    func makeNSView(context: Context) -> NSScrollView {
+        let scrollView = NSScrollView()
+        scrollView.hasVerticalScroller = false
+        scrollView.hasHorizontalScroller = false
+        scrollView.borderType = .noBorder
+        scrollView.drawsBackground = false
+        // Disable user scrolling — scroll is driven by the editor
+        scrollView.verticalScrollElasticity = .none
+        scrollView.horizontalScrollElasticity = .none
+
+        let textView = NSTextView()
+        textView.isEditable = false
+        textView.isSelectable = true
+        textView.isRichText = true
+        textView.usesFontPanel = false
+        textView.drawsBackground = false
+        textView.backgroundColor = .clear
+
+        // Match editor text container settings for alignment
+        textView.textContainer?.lineFragmentPadding = 4
+        textView.textContainerInset = NSSize(width: 8, height: 8)
+
+        // Disable line wrapping to match editor behavior
+        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
+        )
+
+        scrollView.documentView = textView
+        context.coordinator.textView = textView
+        context.coordinator.scrollView = scrollView
+
+        updateContent(textView: textView)
+        return scrollView
+    }
+
+    func updateNSView(_ scrollView: NSScrollView, context: Context) {
+        guard let textView = scrollView.documentView as? NSTextView else { return }
+
+        updateContent(textView: textView)
+
+        // Sync scroll position from editor
+        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)
+        }
+    }
+
+    private func updateContent(textView: NSTextView) {
+        let attributedString = NSMutableAttributedString()
+
+        let paragraphStyle = NSMutableParagraphStyle()
+        paragraphStyle.alignment = .right
+
+        let resultColor = NSColor.secondaryLabelColor
+        let errorColor = NSColor.systemRed
+
+        for (index, lineResult) in results.enumerated() {
+            let displayText = lineResult.result ?? ""
+            let color = lineResult.isError ? errorColor : resultColor
+
+            let attributes: [NSAttributedString.Key: Any] = [
+                .font: font,
+                .foregroundColor: color,
+                .paragraphStyle: paragraphStyle,
+            ]
+
+            let line = NSAttributedString(string: displayText, attributes: attributes)
+            attributedString.append(line)
+
+            // Add newline between lines (but not after the last)
+            if index < results.count - 1 {
+                let newline = NSAttributedString(
+                    string: "\n",
+                    attributes: [.font: font]
+                )
+                attributedString.append(newline)
+            }
+        }
+
+        textView.textStorage?.setAttributedString(attributedString)
+    }
+
+    final class Coordinator {
+        weak var textView: NSTextView?
+        weak var scrollView: NSScrollView?
+    }
+}