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/Sources/CCalcPadEngine/calcpad.h

@@ -0,0 +1,90 @@
+/**
+ * CalcPad Engine — C FFI Header
+ *
+ * This header declares the C-compatible interface for the CalcPad calculation
+ * engine, built in Rust. It is designed for consumption by Swift via a
+ * bridging header or a module map.
+ *
+ * All functions are safe to call from any thread. Panics in Rust are caught
+ * and converted to error results — they never unwind into the caller.
+ *
+ * Memory ownership:
+ *   - Strings returned by calcpad_eval_line / calcpad_eval_sheet are
+ *     heap-allocated by Rust and MUST be freed by calling calcpad_free_result.
+ *   - Passing NULL to calcpad_free_result is a safe no-op.
+ *
+ * JSON schema (version "1.0"):
+ *
+ *   Single-line result (calcpad_eval_line):
+ *   {
+ *     "schema_version": "1.0",
+ *     "result": {
+ *       "value": { "kind": "Number", "value": 42.0 },
+ *       "metadata": {
+ *         "span": { "start": 0, "end": 4 },
+ *         "result_type": "Number",
+ *         "display": "42",
+ *         "raw_value": 42.0
+ *       }
+ *     }
+ *   }
+ *
+ *   Multi-line result (calcpad_eval_sheet):
+ *   {
+ *     "schema_version": "1.0",
+ *     "results": [ ... ]    // array of result objects as above
+ *   }
+ */
+
+#ifndef CALCPAD_H
+#define CALCPAD_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Evaluate a single line of CalcPad input.
+ *
+ * @param input  A null-terminated UTF-8 string containing the expression.
+ *               Passing NULL returns a JSON error result.
+ *
+ * @return A heap-allocated, null-terminated JSON string containing the
+ *         versioned result. The caller MUST free this with
+ *         calcpad_free_result(). Returns NULL only on catastrophic
+ *         allocation failure.
+ */
+char *calcpad_eval_line(const char *input);
+
+/**
+ * Evaluate multiple lines of CalcPad input as a sheet.
+ *
+ * Variable assignments on earlier lines are visible to later lines
+ * (e.g., "x = 5" on line 1 makes x available on line 2).
+ *
+ * @param lines  An array of null-terminated UTF-8 strings.
+ *               NULL entries are treated as empty lines.
+ * @param count  The number of elements in the lines array.
+ *               Must be > 0.
+ *
+ * @return A heap-allocated, null-terminated JSON string containing the
+ *         versioned results array. The caller MUST free this with
+ *         calcpad_free_result(). Returns NULL only on catastrophic
+ *         allocation failure.
+ */
+char *calcpad_eval_sheet(const char *const *lines, int count);
+
+/**
+ * Free a result string previously returned by calcpad_eval_line or
+ * calcpad_eval_sheet.
+ *
+ * @param ptr  The pointer to free. Passing NULL is safe (no-op).
+ *             After this call the pointer is invalid.
+ */
+void calcpad_free_result(char *ptr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* CALCPAD_H */

calcpad-macos/Sources/CCalcPadEngine/module.modulemap

@@ -0,0 +1,5 @@
+module CCalcPadEngine {
+    header "calcpad.h"
+    link "calcpad_engine"
+    export *
+}