Layer system for circuit organization

[darktorres]

Description

Add a layer system that lets users organize complex circuits into visual layers. Elements can be placed on different layers, and layers can be shown/hidden independently. Useful for separating data paths, control logic, and annotations.

Acceptance Criteria

• Multiple named layers supported
• Drag and drop elements between layers
• Per-layer visibility toggle
• Per-layer lock (prevent accidental edits)
• Layers serialized in .panda files

Implementation Guide

1. Layer model

• Add a Layer struct: { int id, QString name, bool visible, bool locked, int zOrder }
• Add QList<Layer> m_layers to Scene
• Each GraphicElement gets a m_layerId property
• Default layer (id=0) for backward compatibility

2. Visibility and locking

• In Scene::drawBackground() and element paint, skip elements on hidden layers
• When a layer is locked, elements on it are not selectable or movable
• Use QGraphicsItem::setFlag(ItemIsSelectable/ItemIsMovable) based on layer lock state

3. Layer panel UI

• Dockable widget listing all layers with visibility eye icon and lock icon
• Right-click: Rename, Delete, Merge layers
• Drag to reorder z-depth
• "Move selection to layer" context menu action

4. Serialization

• Store layer definitions in file metadata
• Store per-element layer assignment in the element's QMap
• Gate with VersionInfo::hasLayers(version) — older versions load everything onto layer 0

Key files

• App/Scene/Scene.h/.cpp — layer storage, visibility filtering
• App/Element/GraphicElement.h/.cpp — layer assignment
• App/Element/GraphicElementSerializer.cpp — serialize layer ID
• App/UI/MainWindow.cpp — layer panel integration
• New files: App/Scene/LayerManager.h/.cpp, App/UI/LayerPanel.h/.cpp