feat(pixel-grid): add explicit grid reconstructor

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

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

backend/native/pixel_grid_reconstructor/.gitignore

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+/target/

backend/native/pixel_grid_reconstructor/Cargo.toml

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+[package]
+name = "windup-pixel-grid-reconstructor"
+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"
+
+[[bin]]
+name = "windup-pixel-grid-reconstructor"
+path = "src/main.rs"
+
+[profile.release]
+lto = true
+codegen-units = 1

backend/native/pixel_grid_reconstructor/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.

backend/native/pixel_grid_reconstructor/README.md

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+# Pixel grid reconstructor
+
+Windup 的独立显式网格重建器。它不检测像素密度，只读取 PNG/JPEG 字节与明确的 `cols`、`rows`、`colors` 参数，并输出对应尺寸的原生 1x PNG。
+
+```bash
+cargo run --release -- --cols 64 --rows 64 --colors 32 < input.png > output.png
+```
+
+模块使用两阶段重建：先对结构色标签投票确定每个格子的归属，再从原图中携带胜出标签的像素恢复颜色。输出的每个像素就是一个规则网格单元，透明度按格内多数票确定。
+
+`colors` 控制结构聚类规模，不是最终图片的强制全局色板上限；这样不会把稀有高光或单像素强调色提前删除。若业务需要固定总色板，应在独立的调色板阶段明确处理。

backend/native/pixel_grid_reconstructor/UPSTREAM.md

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+# Upstream
+
+两阶段重建与 deterministic k-means 逻辑源自 [Retro-Diffusion/pixel-art-fixer](https://github.com/Retro-Diffusion/pixel-art-fixer)，固定于提交 `ef376e57e1c272633ca2dbf5f29ec3fcf6596465`，使用 MIT License。
+
+Windup 将显式规则网格重建提取为独立模块，删除检测、旧重建器和未使用的量化路径，并修复密集网格下负三角权重造成颜色外插的问题。