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

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+# 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`.

parity/cases.toml

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+[meta]
+# Global defaults for all cases.
+width = 1280
+height = 720
+lod = 0
+group = 0
+angle = 0.0
+
+# Per-pixel change threshold for the "changed pixel ratio" metric.
+diff_threshold = 8
+
+# Allowed thresholds (case fails if any limit is exceeded).
+max_mean_abs = 2.0
+max_changed_ratio = 0.010
+
+# Add one block per model.
+#
+# [[case]]
+# id = "animals_a_l_01"
+# archive = "../testdata/Parkan - Iron Strategy/animals.rlb"
+# model = "A_L_01.msh"
+# reference = "reference/animals_a_l_01.png"
+# lod = 0
+# group = 0
+# angle = 0.0
+# max_mean_abs = 2.0
+# max_changed_ratio = 0.010