feat(render-parity): add deterministic frame comparison tool

- Introduced `render-parity` crate for comparing rendered frames against reference images.
- Added command-line options for specifying manifest and output directory.
- Implemented image comparison metrics: mean absolute difference, maximum absolute difference, and changed pixel ratio.
- Created a configuration file `cases.toml` for defining test cases with global defaults and specific parameters.
- Added functionality to capture frames from `render-demo` and save diff images on discrepancies.
- Updated documentation to include usage instructions and CI model for automated testing.

crates/render-parity/README.md

@@ -0,0 +1,16 @@
+# render-parity
+
+Deterministic frame-diff runner for `parkan-render-demo`.
+
+Usage:
+
+```bash
+cargo run -p render-parity -- \
+  --manifest parity/cases.toml \
+  --output-dir target/render-parity/current
+```
+
+Options:
+
+- `--demo-bin <path>`: use prebuilt `parkan-render-demo` binary instead of `cargo run`.
+- `--keep-going`: continue all cases even after failures.