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/error-display.ts

@@ -0,0 +1,142 @@
+/**
+ * Inline error display for CalcPad.
+ * Adapted from epic/9-2-codemirror-6-editor.
+ *
+ * Shows red underline decorations and gutter markers for lines with errors.
+ */
+
+import {
+  Decoration,
+  type DecorationSet,
+  GutterMarker,
+  gutter,
+  EditorView,
+} from '@codemirror/view'
+import { StateField, StateEffect, type Extension, RangeSet } from '@codemirror/state'
+
+// --- State Effects ---
+
+export interface LineError {
+  from: number  // absolute start position in document
+  to: number    // absolute end position in document
+  message: string
+}
+
+export const setErrorsEffect = StateEffect.define<LineError[]>()
+
+// --- Error Gutter Marker ---
+
+class ErrorGutterMarker extends GutterMarker {
+  override toDOM(): HTMLElement {
+    const span = document.createElement('span')
+    span.className = 'cm-error-marker'
+    span.textContent = '\u26A0' // Warning sign
+    return span
+  }
+
+  override eq(other: GutterMarker): boolean {
+    return other instanceof ErrorGutterMarker
+  }
+}
+
+const errorMarkerInstance = new ErrorGutterMarker()
+
+// --- Error Underline Decorations (StateField) ---
+
+const errorUnderlineMark = Decoration.mark({ class: 'cm-error-underline' })
+
+export const errorDecorationsField = StateField.define<DecorationSet>({
+  create() {
+    return Decoration.none
+  },
+
+  update(decos, tr) {
+    for (const effect of tr.effects) {
+      if (effect.is(setErrorsEffect)) {
+        if (effect.value.length === 0) {
+          return Decoration.none
+        }
+        const ranges = effect.value
+          .filter((e) => e.from < e.to)
+          .sort((a, b) => a.from - b.from)
+          .map((e) => errorUnderlineMark.range(e.from, e.to))
+        return RangeSet.of(ranges)
+      }
+    }
+
+    if (tr.docChanged) {
+      return decos.map(tr.changes)
+    }
+
+    return decos
+  },
+
+  provide(field) {
+    return EditorView.decorations.from(field)
+  },
+})
+
+// --- Error Lines Set (for gutter) ---
+
+export const errorLinesField = StateField.define<Set<number>>({
+  create() {
+    return new Set()
+  },
+
+  update(lines, tr) {
+    for (const effect of tr.effects) {
+      if (effect.is(setErrorsEffect)) {
+        const newLines = new Set<number>()
+        for (const error of effect.value) {
+          const lineNumber = tr.state.doc.lineAt(error.from).number
+          newLines.add(lineNumber)
+        }
+        return newLines
+      }
+    }
+    return lines
+  },
+})
+
+// --- Error Gutter ---
+
+export const errorGutter = gutter({
+  class: 'cm-error-gutter',
+  lineMarker(view, line) {
+    const lineNumber = view.state.doc.lineAt(line.from).number
+    const errorLines = view.state.field(errorLinesField)
+    return errorLines.has(lineNumber) ? errorMarkerInstance : null
+  },
+  lineMarkerChange(update) {
+    return update.transactions.some((tr) =>
+      tr.effects.some((e) => e.is(setErrorsEffect)),
+    )
+  },
+})
+
+// --- Base Theme ---
+
+export const errorBaseTheme = EditorView.baseTheme({
+  '.cm-error-underline': {
+    textDecoration: 'underline wavy red',
+  },
+  '.cm-error-marker': {
+    color: '#e53e3e',
+    fontSize: '14px',
+  },
+  '.cm-error-gutter': {
+    width: '20px',
+  },
+})
+
+/**
+ * Creates the error display extension bundle.
+ */
+export function errorDisplayExtension(): Extension {
+  return [
+    errorDecorationsField,
+    errorLinesField,
+    errorGutter,
+    errorBaseTheme,
+  ]
+}