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/Tests/CalcPadTests/PerformanceTests.swift

@@ -0,0 +1,64 @@
+import Testing
+@testable import CalcPad
+
+@Suite("Performance Tests")
+struct PerformanceTests {
+    let engine = StubCalculationEngine()
+
+    @Test("Evaluate 1000+ line sheet completes in under 1 second")
+    func largeSheetEvaluation() async throws {
+        // Generate a 1500-line document mixing expressions, blanks, and comments
+        var lines: [String] = []
+        for i in 1...1500 {
+            switch i % 5 {
+            case 0: lines.append("") // blank
+            case 1: lines.append("// Line \(i)")  // comment
+            case 2: lines.append("\(i) + \(i * 2)")
+            case 3: lines.append("\(i) * 3")
+            case 4: lines.append("\(i) / 7")
+            default: lines.append("\(i)")
+            }
+        }
+        let text = lines.joined(separator: "\n")
+
+        let start = ContinuousClock.now
+        let results = engine.evaluateSheet(text)
+        let elapsed = ContinuousClock.now - start
+
+        #expect(results.count == 1500)
+        #expect(elapsed < .seconds(1), "Sheet evaluation took \(elapsed), expected < 1 second")
+    }
+
+    @Test("Evaluate 5000 line sheet completes in under 3 seconds")
+    func veryLargeSheetEvaluation() async throws {
+        var lines: [String] = []
+        for i in 1...5000 {
+            lines.append("\(i) + \(i)")
+        }
+        let text = lines.joined(separator: "\n")
+
+        let start = ContinuousClock.now
+        let results = engine.evaluateSheet(text)
+        let elapsed = ContinuousClock.now - start
+
+        #expect(results.count == 5000)
+        #expect(elapsed < .seconds(3), "Sheet evaluation took \(elapsed), expected < 3 seconds")
+    }
+
+    @Test("LineResult array construction is efficient for large documents")
+    func lineResultConstruction() {
+        // Verify we get correct line numbering for large documents
+        let lines = (1...1000).map { "\($0) + 1" }
+        let text = lines.joined(separator: "\n")
+        let results = engine.evaluateSheet(text)
+
+        #expect(results.count == 1000)
+        #expect(results.first?.lineNumber == 1)
+        #expect(results.last?.lineNumber == 1000)
+
+        // Spot-check results
+        #expect(results[0].result == "2")   // 1 + 1
+        #expect(results[99].result == "101") // 100 + 1
+        #expect(results[999].result == "1001") // 1000 + 1
+    }
+}