feat: implement bass control (GET/POST /bass)

- Add complete bass control functionality via GET/POST /bass endpoints
- Implement GetBass() for current bass level retrieval
- Add SetBass() with range validation (-9 to +9)
- Include IncreaseBass() and DecreaseBass() with safety limits
- Add SetBassSafe() with automatic value clamping
- Create comprehensive bass models with validation and helpers
- Add CLI flags: -bass, -set-bass, -inc-bass, -dec-bass
- Implement safety features with range validation and clamping
- Create comprehensive test suite (30+ test cases) with mock servers
- Add integration tests with real device validation (SoundTouch 10/20)
- Update documentation with complete BASS-CONTROLS.md guide
- Update API endpoints status (GET/POST /bass: ✅ Implemented)
- Update project status (55% overall completion, 80% control endpoints)
- Real device testing with bass adjustment and validation
- Error handling for invalid ranges and API responses
- XML request/response format validation and compliance
- Human-readable bass level descriptions and categorization

pkg/client/client.go

@@ -296,6 +296,70 @@ func (c *Client) DecreaseVolume(amount int) (*models.Volume, error) {
 	return c.GetVolume()
 }
 
+// GetBass retrieves the current bass level from the /bass endpoint
+func (c *Client) GetBass() (*models.Bass, error) {
+	var bass models.Bass
+	err := c.get("/bass", &bass)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get bass: %w", err)
+	}
+	return &bass, nil
+}
+
+// SetBass sets the bass level using the /bass endpoint
+func (c *Client) SetBass(level int) error {
+	if !models.ValidateBassLevel(level) {
+		return fmt.Errorf("invalid bass level: %d (must be between %d and %d)", level, models.BassLevelMin, models.BassLevelMax)
+	}
+
+	bassReq, err := models.NewBassRequest(level)
+	if err != nil {
+		return fmt.Errorf("failed to create bass request: %w", err)
+	}
+
+	return c.post("/bass", bassReq, nil)
+}
+
+// SetBassSafe sets bass with validation and clamping
+func (c *Client) SetBassSafe(level int) error {
+	clampedLevel := models.ClampBassLevel(level)
+	return c.SetBass(clampedLevel)
+}
+
+// IncreaseBass increases bass by the specified amount (with safety limits)
+func (c *Client) IncreaseBass(amount int) (*models.Bass, error) {
+	currentBass, err := c.GetBass()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get current bass: %w", err)
+	}
+
+	newLevel := models.ClampBassLevel(currentBass.GetLevel() + amount)
+	err = c.SetBass(newLevel)
+	if err != nil {
+		return nil, fmt.Errorf("failed to set bass: %w", err)
+	}
+
+	// Return updated bass
+	return c.GetBass()
+}
+
+// DecreaseBass decreases bass by the specified amount (with safety limits)
+func (c *Client) DecreaseBass(amount int) (*models.Bass, error) {
+	currentBass, err := c.GetBass()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get current bass: %w", err)
+	}
+
+	newLevel := models.ClampBassLevel(currentBass.GetLevel() - amount)
+	err = c.SetBass(newLevel)
+	if err != nil {
+		return nil, fmt.Errorf("failed to set bass: %w", err)
+	}
+
+	// Return updated bass
+	return c.GetBass()
+}
+
 // SelectSource selects an audio source using the /select endpoint
 func (c *Client) SelectSource(source string, sourceAccount string) error {
 	// Validate source parameter