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/build-rust.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+# Build the Rust CalcPad engine for macOS.
+#
+# Produces a static library that the Swift Package Manager links against.
+#
+# Usage:
+#   ./build-rust.sh            # Build release (default)
+#   ./build-rust.sh debug      # Build debug
+#
+# Prerequisites:
+#   - Rust toolchain with targets: aarch64-apple-darwin, x86_64-apple-darwin
+#   - Install targets: rustup target add aarch64-apple-darwin x86_64-apple-darwin
+
+set -euo pipefail
+
+PROFILE="${1:-release}"
+ENGINE_DIR="$(cd "$(dirname "$0")/../calcpad-engine" && pwd)"
+OUTPUT_DIR="$ENGINE_DIR/target/$PROFILE"
+
+if [ "$PROFILE" = "debug" ]; then
+    CARGO_FLAG=""
+else
+    CARGO_FLAG="--release"
+fi
+
+echo "Building calcpad-engine ($PROFILE)..."
+
+# Build for the current architecture
+cd "$ENGINE_DIR"
+cargo build $CARGO_FLAG
+
+echo ""
+echo "Build complete!"
+echo "Static library: $OUTPUT_DIR/libcalcpad_engine.a"
+echo ""
+echo "To build and test the Swift package:"
+echo "  cd calcpad-macos && swift test"