Make backend regressions visible in CI

Issue #295 calls out that core Rust paths have been too dependent on manual app runs. Keep the change narrow by covering existing pure state/audio/clipboard helpers instead of refactoring coordinator or touching hardware-facing code, then wire the existing lib test suite into the lightweight CI lane.

Constraint: Global hotkey, recorder, and insertion integration still require real OS permissions and devices.

Rejected: Split coordinator before adding tests | broader refactor would raise review risk and is tracked separately.

Confidence: high

Scope-risk: narrow

Directive: Keep Rust CI on --lib unless integration tests get explicit OS fixtures.

Tested: npm run build

Tested: cargo test --manifest-path src-tauri/Cargo.toml --lib

Tested: cargo check --manifest-path src-tauri/Cargo.toml

Not-tested: Live microphone, global hotkey, clipboard, and insertion device paths.

Related: #295

.github/workflows/ci.yml

@@ -91,6 +91,9 @@ jobs:
       - name: Check Tauri backend (cargo check)
         run: cargo check --manifest-path src-tauri/Cargo.toml
 
+      - name: Run Rust backend unit tests
+        run: cargo test --manifest-path src-tauri/Cargo.toml --lib
+
       - name: Verify version sync across all 5 files
         # 两个平台都跑这个校验：Windows runner 自带 git-bash，跨 shell 表现一致。
         # 一旦版本号 drift 立刻 fail，避免发版时再发现漏改。