Copy/Paste/Cut operations

[Red-Cat-Fat]

Motivation

• Provide basic clipboard operations for serialize-reference fields so values (references) can be moved or shared between fields.
• Ensure copy/paste semantics preserve the original reference (not deep-clone) so mutations affect all pasted locations.

Description

• Added a simple editor clipboard SRClipboard that holds a single ManagedReferenceValue and exposes HasValue (SerializeReferenceEditor/Assets/SREditor/Package/Editor/Scripts/SRActions/SRClipboard.cs).
• Implemented Copy, Paste, and Cut actions as CopyPropertySRAction, PastePropertySRAction, and CutPropertySRAction respectively, where Copy stores the reference, Paste assigns the stored reference to the target field, and Cut stores the reference then clears the source field (SerializeReferenceEditor/Assets/SREditor/Package/Editor/Scripts/SRActions/CopyPropertySRAction.cs, PastePropertySRAction.cs, CutPropertySRAction.cs).
• Added Undo support for destructive/applying operations in Paste and Cut and applied modified properties after changes.
• Wired the new actions into the action factory and the search window menu so they appear alongside Erase (SRActionFactory.cs and SRTypesSearchWindowProvider.cs).