refactor: split Linux egui host from shared backend

[H-Chris233]

Summary

• 提取无 Tauri/egui 依赖的 openless-core，集中共享业务、状态机、provider、持久化、DTO、错误和语义事件。
• 保留 Tauri 作为 macOS/Windows/Android 薄适配器；Linux 使用独立 openless-linux-egui host，不编译 Tauri/WebKitGTK。
• 迁移 Core unit/contract tests，删除旧的 Tauri path-include 测试旁路，并增加 Tauri/Linux source and host contracts。
• 提供 egui 团队使用的公共接口、headless example、fixtures、能力矩阵、事件映射和接入文档。
• 拆分 Linux 验证打包 workflow，并修复 Tauri updater Rust/JS 版本约束漂移。

Validation

• Final CI (head 297c5182): https://github.com/H-Chris233/openless/actions/runs/33392385606
• Tauri artifacts (head 4632e09c): https://github.com/H-Chris233/openless/actions/runs/33389007087
• Android debug APK artifacts (head 4632e09c): https://github.com/H-Chris233/openless/actions/runs/33389007792
• Linux deb/rpm/AppImage artifact and manifest SHA-256 verification are included in the CI runs above.
• shared-backend-wire-contract.test.mjs passed.

Scope notes

• egui/eframe UI implementation and visual/UI acceptance remain with the egui team.
• Formal signing, real-device installation, Ubuntu desktop input/audio smoke, and release publication remain separate release gates.