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-windows/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "calcpad-windows"
+version = "0.1.0"
+edition = "2021"
+description = "CalcPad — native Windows app built with iced"
+
+[[bin]]
+name = "calcpad-win"
+path = "src/main.rs"
+
+[dependencies]
+calcpad-engine = { path = "../calcpad-engine" }
+iced = { version = "0.13", features = ["tokio"] }
+tokio = { version = "1", features = ["rt-multi-thread", "macros"] }
+
+[profile.release]
+opt-level = 3
+lto = true
+strip = true