chore: 整理仓库 — 版本号统一、README 徽章更新、新增 CHANGELOG 和 CONTRIBUTING

- 版本号 4.0.0 → 4.2.0（tauri.conf.json, Cargo.toml, 窗口标题）
- README 徽章更新（tests 55→72, version 4.0.0→4.2.0）
- CHANGELOG.md 补充 v4.1 和 v4.2 变更记录
- 新增 CONTRIBUTING.md 贡献指南
- GitHub Release: v4.2.0 补充说明，旧 C 版本标记为 pre-release

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "patheditor"
-version = "4.0.0"
+version = "4.2.0"
 description = "Windows PATH Environment Variable Editor"
 authors = ["刘航宇"]
 license = "MIT"

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "../node_modules/@tauri-apps/cli/config.schema.json",
   "productName": "PathEditor",
-  "version": "4.0.0",
+  "version": "4.2.0",
   "identifier": "com.liuhangyu.patheditor",
   "build": {
     "frontendDist": "../dist",
@@ -12,7 +12,7 @@
   "app": {
     "windows": [
       {
-        "title": "PathEditor v4.0",
+        "title": "PathEditor v4.2",
         "width": 900,
         "height": 700,
         "minWidth": 800,