Mat4x4 row labels, no scientific notation, per-component inline editing

- Add row0..row3 labels to Mat4x4 grid display with aligned columns
- Rewrite fmtFloat() to never use scientific notation (plain decimal + trim)
- Enable per-component inline editing for all 16 Mat4x4 floats
- Fix click-to-edit always selecting first component (thread column from hitTestTarget)
- Add isMatrixKind() helper, remove Mat4x4 from context menu edit exclusion

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

src/core.h

@@ -134,6 +134,9 @@ inline constexpr bool isHexNode(NodeKind k) {
 inline constexpr bool isVectorKind(NodeKind k) {
     return k == NodeKind::Vec2 || k == NodeKind::Vec3 || k == NodeKind::Vec4;
 }
+inline constexpr bool isMatrixKind(NodeKind k) {
+    return k == NodeKind::Mat4x4;
+}
 
 inline QStringList allTypeNamesForUI(bool stripBrackets = false) {
     QStringList out;