Add command-based undo/redo system

- New renzora_undo crate with UndoCommand trait + UndoStacks resource
- Ctrl+Z / Ctrl+Y / Ctrl+Shift+Z shortcuts, per-context stacks
- Built-in commands: spawn/delete shapes, transform, rename, reparent,
  lock/visibility toggle, field edit, spawn preset/component, group,
  hierarchy reorder, compound
- Wired: shape library, viewport drop, hierarchy (rename/delete/lock/
  visibility/reparent/group/reorder/add overlay), gizmo drag, modal
  G/R/S transform, delete key, inspector fields
- Exposed via renzora SDK (renzora::undo)

Author: RenzoraEngine

crates/core/renzora_undo/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "renzora_undo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+bevy = { workspace = true }
+renzora_core = { path = "../renzora_core" }
+renzora_editor_framework = { path = "../../editor/renzora_editor" }
+log = "0.4"