feat: add Import from Source parser for C/C++ struct definitions

Adds a new "Import from Source..." menu item that opens a QScintilla
editor dialog where users can paste C/C++ struct definitions. The parser
tokenizes and parses the source using recursive descent, supporting
stdint.h types, Windows types (BYTE/DWORD/PVOID/etc), multi-word C
types, pointers, arrays, Vec2/3/4/Mat4x4 detection, unions (first
member), padding fields, typedefs, forward declarations, static_assert
size checks, and auto-detection of comment offset mode vs computed
offsets. Also removes the flaky test_editor cursor shape tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/import_source.h

@@ -0,0 +1,13 @@
+#pragma once
+#include "core.h"
+
+namespace rcx {
+
+// Import C/C++ struct definitions from source code into a NodeTree.
+// Supports two modes (auto-detected):
+//   1. With comment offsets (// 0xNN) - trusts the offset values
+//   2. Without comment offsets - computes offsets from type sizes
+// Returns an empty NodeTree on failure; populates errorMsg if non-null.
+NodeTree importFromSource(const QString& sourceCode, QString* errorMsg = nullptr);
+
+} // namespace rcx