docs(avalonia): add Phase 9 testing guide

Author: lookbusy1344

avalonia-gui/TESTING_GUIDE.md

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+# Phase 9 Testing Guide
+
+This guide covers testing the file operations and dialogs implemented in Phase 9.
+
+## Prerequisites
+
+- .NET 10 SDK installed
+- Go backend built (`make build` in project root)
+- macOS 26.2 (primary development platform)
+
+## Build & Run
+
+```bash
+cd avalonia-gui
+
+# Build
+dotnet build
+
+# Run application
+dotnet run --project ARMEmulator
+
+# Run tests
+dotnet test
+```
+
+## Test Checklist
+
+### 1. File Menu Commands
+
+- [ ] **Open File (Ctrl+O)**
+  - Click File > Open or press Ctrl+O
+  - Select an assembly file (.s)
+  - Verify file content loads in editor
+  - Verify file is added to Recent Files
+
+- [ ] **Save File (Ctrl+S)**
+  - Make changes to editor
+  - Click File > Save or press Ctrl+S
+  - Verify file is saved to current location
+  - For new files, should show Save As dialog
+
+- [ ] **Save As (Ctrl+Shift+S)**
+  - Click File > Save As or press Ctrl+Shift+S
+  - Enter new filename
+  - Verify file is saved to new location
+  - Verify file is added to Recent Files
+
+- [ ] **Recent Files**
+  - Open several files
+  - Click File > Recent Files
+  - Verify list shows recently opened files
+  - Click a recent file to open it
+  - Verify file loads correctly
+
+- [ ] **Examples Browser (Ctrl+Shift+E)**
+  - Click File > Examples or press Ctrl+Shift+E
+  - Search for examples (try "fib", "hello", etc.)
+  - Verify search filters list
+  - Select an example
+  - Verify preview shows on right
+  - Click Load
+  - Verify example loads in editor
+
+- [ ] **Preferences (Ctrl+,)**
+  - Click File > Preferences or press Ctrl+,
+  - **General Tab:**
+    - Modify Backend URL
+    - Change Theme (Auto/Light/Dark)
+    - Adjust Recent Files Limit
+    - Toggle Auto-scroll memory writes
+  - **Editor Tab:**
+    - Adjust Font Size (10-24)
+    - Verify live preview updates
+  - Click OK
+  - Verify settings are applied (font size should change)
+
+- [ ] **About Dialog**
+  - Click File > About
+  - Verify app version displays
+  - Verify backend version loads (if backend is running)
+  - If backend not running, should show "Not available"
+  - Verify commit hash and build date appear
+  - Click Close
+
+### 2. Keyboard Shortcuts
+
+Verify all keyboard shortcuts work:
+- Ctrl+O: Open
+- Ctrl+S: Save
+- Ctrl+Shift+S: Save As
+- Ctrl+Shift+E: Examples
+- Ctrl+,: Preferences
+
+### 3. Dialog Behavior
+
+- [ ] All dialogs center on parent window
+- [ ] Dialogs are modal (parent disabled while open)
+- [ ] Cancel closes without applying changes
+- [ ] OK/Load buttons apply changes
+
+### 4. File Service Integration
+
+- [ ] Opening file updates editor content
+- [ ] Opening file auto-loads program
+- [ ] Saving file preserves content
+- [ ] Recent files list updates correctly
+- [ ] Recent files persists across sessions (TODO: needs persistence implementation)
+
+### 5. Error Handling
+
+- [ ] Try opening non-existent file from Recent Files
+  - Should show error message
+  - Should not crash
+- [ ] Try opening invalid assembly file
+  - Should load but show parse errors
+- [ ] Try examples with backend not running
+  - Should show error in examples browser
+
+## Platform-Specific Testing
+
+### macOS
+- [ ] Native file dialogs appear
+- [ ] Menu bar integrates with system
+- [ ] Keyboard shortcuts follow macOS conventions
+- [ ] App bundle structure correct
+
+### Windows (if available)
+- [ ] Native file dialogs appear
+- [ ] Menu bar appears in window
+- [ ] Keyboard shortcuts work
+- [ ] .exe runs standalone
+
+### Linux (if available)
+- [ ] File dialogs work
+- [ ] Menu bar appears
+- [ ] Keyboard shortcuts work
+
+## Known Limitations
+
+1. **Settings Persistence**: Not yet implemented. Settings reset on restart.
+2. **Recent Files Persistence**: Not yet implemented. List clears on restart.
+3. **Theme Changes**: Require app restart to take effect.
+
+## Test Results
+
+All 223 unit tests passing:
+- AppSettings: 6 tests
+- FileService: 8 tests
+- AboutWindowViewModel: 5 tests
+- PreferencesWindowViewModel: 6 tests
+- ExamplesBrowserViewModel: 6 tests
+- MainWindowViewModel: 10 tests
+- Other ViewModels and Services: 182 tests
+
+Build status: ✅ Clean build with 0 warnings, 0 errors