docs: add Lingow detailed module design

[torbys]

补充 Lingow 整体模块架构、职责边界与关键接口的详细设计文档。

[github-actions]

Go coverage

Scope	Current	Baseline	Delta
packages/contracts	86.4%	86.4%	+0.0%
services/api	55.2%	55.2%	-0.0%
services/realtime-audio	77.4%	77.4%	-0.0%
Total	63.7%	63.7%	-0.0%

Files below baseline

Showing the 5 largest file-level regressions.

File	Current	Baseline	Delta
services/api/github.com/1024XEngineer/xe6-tsy/services/api/internal/delivery/provider.go	98.6%	100.0%	-1.4%
services/realtime-audio/github.com/1024XEngineer/xe6-tsy/services/realtime-audio/asr/qwen/provider.go	71.1%	71.6%	-0.5%

Base branch: dev · Commit: 7871e75 · Test workflow run