refactor(legacy): Create engine/legacy/ subsystem and move render/layers (Phase 3.2)

- Create engine/legacy/ package for deprecated rendering modules
- Move engine/render.py → engine/legacy/render.py (274 lines)
- Move engine/layers.py → engine/legacy/layers.py (272 lines)
- Update engine/legacy/layers.py to import from engine.legacy.render
- Add comprehensive package documentation
- Tests will be updated in next commit (Phase 3.3)

engine/legacy/__init__.py

@@ -0,0 +1,15 @@
+"""
+Legacy rendering modules for backwards compatibility.
+
+This package contains deprecated rendering code from the old pipeline architecture.
+These modules are maintained for backwards compatibility with adapters and tests,
+but should not be used in new code.
+
+New code should use the Stage-based pipeline architecture instead.
+
+Modules:
+- render: Legacy font/gradient rendering functions
+- layers: Legacy layer compositing and effect application
+
+All modules in this package are marked deprecated and will be removed in a future version.
+"""