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.

parity/README.md

@@ -0,0 +1,20 @@
+# Render Parity Dataset
+
+This folder stores parity-test input for `crates/render-parity`.
+
+- `cases.toml`: list of deterministic render cases.
+- `reference/*.png`: baseline frames captured from the original renderer.
+
+Expected workflow:
+
+1. Capture baseline PNG frames from original game/editor for each case.
+2. Add entries to `cases.toml`.
+3. Run:
+
+```bash
+cargo run -p render-parity -- \
+  --manifest parity/cases.toml \
+  --output-dir target/render-parity/current
+```
+
+On failure, diff images are saved to `target/render-parity/current/diff`.