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-web/src/editor/CalcEditor.tsx

@@ -0,0 +1,224 @@
+/**
+ * React wrapper around CodeMirror 6 for the CalcPad editor.
+ *
+ * Integrates the CalcPad language mode, answer gutter, error display,
+ * and debounced evaluation via the WASM engine Web Worker.
+ */
+
+import { useRef, useEffect, useCallback } from 'react'
+import { EditorState } from '@codemirror/state'
+import {
+  EditorView,
+  lineNumbers,
+  drawSelection,
+  highlightActiveLine,
+  keymap,
+} from '@codemirror/view'
+import {
+  defaultHighlightStyle,
+  syntaxHighlighting,
+  bracketMatching,
+  indentOnInput,
+} from '@codemirror/language'
+import { defaultKeymap, history, historyKeymap } from '@codemirror/commands'
+import { calcpadLanguage } from './calcpad-language.ts'
+import { answerGutterExtension, setAnswersEffect, type LineAnswer } from './answer-gutter.ts'
+import { errorDisplayExtension, setErrorsEffect, type LineError } from './error-display.ts'
+import type { EngineLineResult } from '../engine/types.ts'
+
+export interface CalcEditorProps {
+  /** Initial document content */
+  initialDoc?: string
+  /** Called when the document text changes (debounced internally) */
+  onDocChange?: (lines: string[]) => void
+  /** Engine evaluation results to display in the answer gutter */
+  results?: EngineLineResult[]
+  /** Debounce delay in ms before triggering onDocChange */
+  debounceMs?: number
+}
+
+/**
+ * CalcPad editor component built on CodeMirror 6.
+ * Handles syntax highlighting, line numbers, answer gutter,
+ * and error underlines.
+ */
+export function CalcEditor({
+  initialDoc = '',
+  onDocChange,
+  results,
+  debounceMs = 50,
+}: CalcEditorProps) {
+  const containerRef = useRef<HTMLDivElement>(null)
+  const viewRef = useRef<EditorView | null>(null)
+  const timerRef = useRef<ReturnType<typeof setTimeout> | null>(null)
+
+  // Stable callback ref for doc changes
+  const onDocChangeRef = useRef(onDocChange)
+  onDocChangeRef.current = onDocChange
+
+  const scheduleEval = useCallback((view: EditorView) => {
+    if (timerRef.current) clearTimeout(timerRef.current)
+    timerRef.current = setTimeout(() => {
+      timerRef.current = null
+      const doc = view.state.doc.toString()
+      const lines = doc.split('\n')
+      onDocChangeRef.current?.(lines)
+    }, debounceMs)
+  }, [debounceMs])
+
+  // Create editor on mount
+  useEffect(() => {
+    if (!containerRef.current) return
+
+    const updateListener = EditorView.updateListener.of((update) => {
+      if (update.docChanged && viewRef.current) {
+        scheduleEval(viewRef.current)
+      }
+    })
+
+    const state = EditorState.create({
+      doc: initialDoc,
+      extensions: [
+        lineNumbers(),
+        drawSelection(),
+        highlightActiveLine(),
+        bracketMatching(),
+        indentOnInput(),
+        history(),
+        keymap.of([...defaultKeymap, ...historyKeymap]),
+        syntaxHighlighting(defaultHighlightStyle),
+        calcpadLanguage(),
+        answerGutterExtension(),
+        errorDisplayExtension(),
+        updateListener,
+        calcpadEditorTheme,
+      ],
+    })
+
+    const view = new EditorView({
+      state,
+      parent: containerRef.current,
+    })
+
+    viewRef.current = view
+
+    // Trigger initial evaluation
+    const doc = view.state.doc.toString()
+    onDocChangeRef.current?.(doc.split('\n'))
+
+    return () => {
+      if (timerRef.current) clearTimeout(timerRef.current)
+      view.destroy()
+      viewRef.current = null
+    }
+    // initialDoc intentionally excluded — we only set it once on mount
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [scheduleEval])
+
+  // Push engine results into the answer gutter + error display
+  useEffect(() => {
+    const view = viewRef.current
+    if (!view || !results) return
+
+    const answers: LineAnswer[] = []
+    const errors: LineError[] = []
+
+    for (let i = 0; i < results.length; i++) {
+      const lineNum = i + 1
+      const result = results[i]
+      answers.push({ line: lineNum, result })
+
+      if (result.type === 'error' && result.error) {
+        // Map to document positions
+        if (lineNum <= view.state.doc.lines) {
+          const docLine = view.state.doc.line(lineNum)
+          errors.push({
+            from: docLine.from,
+            to: docLine.to,
+            message: result.error,
+          })
+        }
+      }
+    }
+
+    view.dispatch({
+      effects: [
+        setAnswersEffect.of(answers),
+        setErrorsEffect.of(errors),
+      ],
+    })
+  }, [results])
+
+  return <div ref={containerRef} className="calc-editor" />
+}
+
+/**
+ * Base theme for the CalcPad editor.
+ */
+const calcpadEditorTheme = EditorView.baseTheme({
+  '&': {
+    height: '100%',
+    fontSize: '15px',
+    fontFamily: 'ui-monospace, Consolas, "Courier New", monospace',
+  },
+  '.cm-scroller': {
+    overflow: 'auto',
+  },
+  '.cm-content': {
+    padding: '12px 0',
+    minHeight: '100%',
+  },
+  '.cm-line': {
+    padding: '0 16px',
+    lineHeight: '1.6',
+  },
+  '.cm-gutters': {
+    backgroundColor: 'transparent',
+    borderRight: 'none',
+  },
+  '.cm-lineNumbers .cm-gutterElement': {
+    padding: '0 8px 0 16px',
+    color: '#9ca3af',
+    fontSize: '13px',
+    minWidth: '32px',
+  },
+  '.cm-answer-gutter': {
+    minWidth: '140px',
+    textAlign: 'right',
+    paddingRight: '16px',
+    borderLeft: '1px solid #e5e4e7',
+    backgroundColor: '#f8f9fa',
+    fontFamily: 'ui-monospace, Consolas, monospace',
+    fontSize: '14px',
+  },
+  '&dark .cm-answer-gutter': {
+    borderLeft: '1px solid #2e303a',
+    backgroundColor: '#1a1b23',
+  },
+  '.cm-answer-value': {
+    color: '#6366f1',
+    fontWeight: '600',
+  },
+  '&dark .cm-answer-value': {
+    color: '#818cf8',
+  },
+  '.cm-answer-error': {
+    color: '#e53e3e',
+    fontStyle: 'italic',
+  },
+  '&dark .cm-answer-error': {
+    color: '#fc8181',
+  },
+  '.cm-activeLine': {
+    backgroundColor: 'rgba(99, 102, 241, 0.04)',
+  },
+  '&dark .cm-activeLine': {
+    backgroundColor: 'rgba(129, 140, 248, 0.06)',
+  },
+  '.cm-selectionBackground': {
+    backgroundColor: 'rgba(99, 102, 241, 0.15) !important',
+  },
+  '.cm-focused': {
+    outline: 'none',
+  },
+})