[T1] Move structural_renderer from mingling_core to mingling
Breaking: inject StructuralRenderer via Hook instead of hardcoding it into the core loop
> [!NOTE]
>
> This is a **Breaking Change** planned for Mingling 0.5.0.
## Background
Mingling's Hook system is now complete, so there's no longer a need to hardcode `StructuralRenderer` into the core loop.
The plan is to remove it from `exec.rs` and instead inject the Hook implementation via `StructuralRendererSetup`.
## Plan
- Remove the hardcoded `StructuralRenderer` from the core execution loop (`exec.rs`).
- Implement the renderer as a Hook and inject it via `StructuralRendererSetup`.
- `mingling_core` no longer depends on the structural renderer; the wiring moves up to the `mingling` crate level.
## Tasks
- [ ] Audit where `StructuralRenderer` is hardcoded in `mingling_core` (`exec.rs` and related)
- [ ] Design `StructuralRendererSetup` as a Hook implementation
- [ ] Move / re-implement the renderer wiring in `mingling`
- [ ] Clean up `mingling_core`'s `structural_renderer` feature and its serde deps (if no longer needed there)
- [ ] Migrate examples and tests
- [ ] Verify structural renderer output is unchanged
## 🕘 Progress
- [ ] In Progress
- [ ] Complete
Written by @Weicao-CatilGrass