refactor: split canvas-rs into canvas and images workspace crates (zero external dependencies)

[echosoar]

Reorganises the project as a Cargo workspace with two separate crates, both with zero external dependencies:

Changes Made

• canvas crate — core 2D drawing library that operates entirely on an in-memory RGBA pixel buffer. No external dependencies. to_data_url and to_blob have been removed from Canvas.
• images crate — PNG encoding and decoding implemented entirely in pure Rust with no external dependencies. Exposes:
  • images::to_data_url(canvas: &Canvas) -> String — encodes a canvas as a data:image/png;base64,… URL
  • images::to_blob(canvas: &Canvas) -> Vec<u8> — returns raw PNG bytes for a canvas
  • images::from_png(bytes: &[u8]) -> Result<ImageData, String> — decodes a PNG byte slice into RGBA ImageData using a pure-Rust DEFLATE/zlib decoder

Both PNG encoding (zlib stored-block DEFLATE) and PNG decoding (zlib inflate + PNG filter reconstruction) are implemented from scratch with no third-party crates.

Testing

• ✅ All tests pass across both crates
• ✅ Integration tests moved to images/tests/ and updated to use the new images::to_data_url, images::to_blob, and images::from_png API
• ✅ Round-trip encode→decode verified for correctness

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[echosoar]

@copilot
把项目分为两个 crates，一个是 canvas，提供核心的能力，所有的处理都是基于 rgba 的颜色矩阵；另外一个是 images，提供 png 的decoding 和 encoding，可以把 rgba 矩阵转换为png图片，或者是把 png图片转换为 rgba 数据，所以 toDataUrl 和 toBlob 方法从 canvas 踢出，变成 images crate的方法，参数为输入一个canvas实例

[Copilot]

@copilot
把项目分为两个 crates，一个是 canvas，提供核心的能力，所有的处理都是基于 rgba 的颜色矩阵；另外一个是 images，提供 png 的decoding 和 encoding，��...

Done in cf5455e. The repo is now a Cargo workspace with two crates:

• canvas — pure-Rust core drawing library, operates entirely on an RGBA pixel buffer, no external dependencies. to_data_url and to_blob have been removed from Canvas.
• images — PNG encoding/decoding, depends on canvas and the png crate. Exposes:
  • images::to_data_url(canvas: &Canvas) -> String
  • images::to_blob(canvas: &Canvas) -> Vec<u8>
  • images::from_png(bytes: &[u8]) -> Result<ImageData, String>

Integration tests have been moved to images/tests/ and updated to use the new API.

[echosoar]

@copilot

我无法使用任何外部依赖，所以 images 里面无法依赖png库，png的encode 和decode 都需要源码重新实现