docs(plan): record Win32-opacity decision

Replaces the eval-based set_window_opacity in the plan with the
SetLayeredWindowAttributes implementation that actually shipped.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/plans/2026-05-08-browserlay-mvp.md

@@ -1883,8 +1883,9 @@ pub fn get_hotkey_status(app: AppHandle) -> Result<crate::HotkeyStatus, String>
     Ok(guard.clone())
 }
 
-/// Set window opacity. Tauri 2's JS WebviewWindow does NOT expose setOpacity
-/// (only the Rust side does), so the JS layer routes through this command.
+/// Set window opacity. Tauri 2.11 does NOT expose set_opacity on either side
+/// (open issue tauri-apps/tauri#3279), so we drop down to Win32 directly via
+/// SetLayeredWindowAttributes. See `apply_window_opacity` for details.
 #[tauri::command]
 pub fn set_window_opacity<R: Runtime>(
     app: AppHandle<R>,
@@ -1894,10 +1895,12 @@ pub fn set_window_opacity<R: Runtime>(
     let Some(window) = app.get_webview_window(&label) else {
         return Err(format!("window not found: {label}"));
     };
-    window.set_opacity(opacity).map_err(|e| e.to_string())
+    apply_window_opacity(&window, opacity)
 }
 ```
 
+> **Window opacity implementation note:** Tauri 2.11.x's `WebviewWindow` doesn't expose `set_opacity` on either the JS or Rust side (long-running open issue tauri-apps/tauri#3279). To get true OS-level window translucency we drop down to the Win32 API directly. Pin `windows = "0.61"` in `Cargo.toml` to match Tauri's transitive `windows` version (otherwise `WebviewWindow::hwnd()` returns an HWND from a different `windows` major and you get a diamond-dependency type mismatch). The `apply_window_opacity` helper sets `WS_EX_LAYERED` if not already present, then calls `SetLayeredWindowAttributes(hwnd, 0, alpha, LWA_ALPHA)`.
+
 > **Note on the design:** Tauri 2 doesn't expose a getter for `ignore_cursor_events` / `opacity` on `WebviewWindow`. Rather than rely on getters, we keep a tiny Rust-side cache (`AppState.click_through`) that the JS side keeps in sync via `sync_click_through_cache`. The global shortcut handler reads this cache, computes the inverse, calls `apply_state`. The JS-driven toggle path uses `toggle_click_through(current)` and skips the cache.
 
 - [ ] **Step 3: Commit**