feat: Add Z-Image ControlNet support with spatial conditioning

Add comprehensive ControlNet support for Z-Image models including:

Backend:
- New ControlNet_Checkpoint_ZImage_Config for Z-Image control adapter models
- Z-Image control key detection (_has_z_image_control_keys) to identify control layers
- ZImageControlAdapter loader for standalone control models
- ZImageControlTransformer2DModel combining base transformer with control layers
- Memory-efficient model loading by building combined state dict

VRAM usage is high.

- Auto-detect control_in_dim from adapter weights (16 for V1, 33 for V2.0)
- Auto-detect n_refiner_layers from state dict
- Add zero-padding for V2.0's additional channels
- Use accelerate.init_empty_weights() for efficient model creation
- Add ControlNet_Checkpoint_ZImage_Config to frontend schema

- Add missing ControlNet_Checkpoint_ZImage_Config import
- Remove unused imports (Any, Dict, ADALN_EMBED_DIM, is_torch_version)
- Add strict=True to zip() calls
- Replace mutable list defaults with immutable tuples
- Replace dict() calls with literal syntax
- Sort imports in z_image_denoise.py

Implement Z-Image ControlNet as an Extension pattern (similar to FLUX ControlNet)
instead of merging control weights into the base transformer. This provides:
- Lower memory usage (no weight duplication)
- Flexibility to enable/disable control per step
- Cleaner architecture with separate control adapter

Key implementation details:
- ZImageControlNetExtension: computes control hints per denoising step
- z_image_forward_with_control: custom forward pass with hint injection
- patchify_control_context: utility for control image patchification
- ZImageControlAdapter: standalone adapter with control_layers and noise_refiner

Architecture matches original VideoX-Fun implementation:
- Hints computed ONCE using INITIAL unified state (before main layers)
- Hints injected at every other main transformer layer (15 control blocks)
- Control signal added after each designated layer's forward pass

V2.0 ControlNet support (control_in_dim=33):
- Channels 0-15: control image latents
- Channels 16-31: reference image (zeros for pure control)
- Channel 32: inpaint mask (1.0 = don't inpaint, use control signal)