Remove ambient validation markers, show ptr64 as void*, add struct types to picker, add README

- Remove M_ERR and M_PTR0 ambient markers from compose (validation only during inline edit)
- Change ptr64 display name to void* in kKindMeta
- Type picker now includes custom struct type names from the tree
- Controller handles struct type selection from picker
- Add project README

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

src/editor.h

@@ -41,6 +41,9 @@ public:
     void setEditorFont(const QString& fontName);
     static void setGlobalFontName(const QString& fontName);
 
+    // Custom type names (struct types from the tree) shown in type picker
+    void setCustomTypeNames(const QStringList& names) { m_customTypeNames = names; }
+
 signals:
     void marginClicked(int margin, int line, Qt::KeyboardModifiers mods);
     void contextMenuRequested(int line, int nodeIdx, int subLine, QPoint globalPos);
@@ -102,6 +105,9 @@ private:
     // ── Tab cycling state ──
     EditTarget m_lastTabTarget = EditTarget::Value;
 
+    // ── Custom type names for type picker ──
+    QStringList m_customTypeNames;
+
     // ── Reentrancy guards ──
     bool m_clampingSelection = false;
     bool m_updatingComment = false;