docs: 开源规范化 — doc comments + 社区文件 + 示例代码 + crates.io 就绪

- 为 core 公开 API 添加完整 doc comments（rustdoc 可用）
- 新增 .editorconfig / CONTRIBUTING / CODE_OF_CONDUCT / SECURITY
- 新增 Issue 模板（bug + feature）+ PR 模板
- 新增 3 个代码示例（examples/）
- 更新 Cargo.toml 元数据（description/repository/keywords/categories/MSRV）
- 更新 README + CHANGELOG

Cargo.toml

@@ -6,4 +6,12 @@ members = ["core", "cli", "gui", "web"]
 version = "0.1.0"
 edition = "2021"
 license = "MIT"
-authors = ["刘航宇"]
+authors = ["刘航宇 <lhy3364451258@163.com>"]
+description = "从零手写的 QR 码生成器 — ISO/IEC 18004 完整实现"
+repository = "https://github.com/LHY0125/QRGen"
+homepage = "https://github.com/LHY0125/QRGen"
+documentation = "https://github.com/LHY0125/QRGen"
+readme = "README.md"
+keywords = ["qr", "qr-code", "qrcode", "encoding", "barcode"]
+categories = ["encoding", "command-line-utilities", "gui", "web-programming"]
+rust-version = "1.87"