feat(pixel-grid): add independent native grid detector

.github/workflows/pixel-grid-detector.yml

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+name: Pixel grid detector CI
+
+on:
+  push:
+    paths:
+      - "native/pixel-perfect/**"
+      - ".github/workflows/pixel-grid-detector.yml"
+  pull_request:
+    paths:
+      - "native/pixel-perfect/**"
+      - ".github/workflows/pixel-grid-detector.yml"
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: native/pixel-perfect/crates/detector
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Rust format
+        run: cargo fmt --check
+
+      - name: Rust tests
+        run: cargo test --release --locked

native/pixel-perfect/.gitignore

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+target/

native/pixel-perfect/crates/detector/Cargo.toml

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+[package]
+name = "windup-pixel-grid-detector"
+version = "0.1.0"
+edition = "2021"
+license = "MIT"
+publish = false
+
+[dependencies]
+image = { version = "0.25", default-features = false, features = ["png", "jpeg"] }
+rayon = "1.12.0"
+rustfft = "6"

native/pixel-perfect/crates/detector/LICENSE

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+MIT License
+
+Copyright (c) 2026 Astropulse, LLC
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

native/pixel-perfect/crates/detector/README.md

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+# Pixel grid detector
+
+Windup 的独立本地像素网格识别库。它只读取 PNG/JPEG 字节并返回网格元数据，不负责重建图片，也不引用任何生成管线或应用服务。
+
+默认执行 Pixel Art Fixer 的完整多检测器共识流程：autocorrelation、run-length comb、shift self-similarity 与证据仲裁。调用方可以显式选择 `DetectorMode::Fast` 作为低延迟模式。
+
+```bash
+cargo test --release --locked
+```
+
+公共入口为 `detect_bytes`，返回字段包括 `cols`、`rows`、`step_x`、`step_y`、`consensus` 和 `confidence`。Python 绑定由独立集成模块提供，本 crate 不依赖 Python 或 Windup 后端。
+
+## Capability boundary
+
+传统网格检测默认面向隐含像素单元不小于约 3px 的输入。小于 3px 的高密度伪像素可能缺少足够的周期证据，本模块暂不保证其自动识别结果；调用方应允许用户提供显式像素尺寸。这里不添加针对单张图片的启发式特例。

native/pixel-perfect/crates/detector/UPSTREAM.md

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+# Upstream
+
+检测算法源自 [Retro-Diffusion/pixel-art-fixer](https://github.com/Retro-Diffusion/pixel-art-fixer)，固定于提交 `ef376e57e1c272633ca2dbf5f29ec3fcf6596465`，使用 MIT License。
+
+Windup 仅增加字节输入的 library 边界、输入资源限制、项目内测试与统一格式化；检测算法逻辑保持上游实现。
+
+`tests/frog-500.png` 由同一固定提交的 MIT 示例 `examples/frog.png` 以 Lanczos 缩放至 500×500，用于覆盖完整检测器的仲裁路径。