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/public/icons/icon-512.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" rx="64" fill="#6366f1"/>
+  <text x="256" y="340" text-anchor="middle" font-family="system-ui, sans-serif" font-weight="700" font-size="260" fill="white">=</text>
+</svg>