refactor: group viewport/zoom/pan state into a Viewport sub-struct

The visible-window + pan-anchor state was 10 flat fields (viewStart, viewEnd,
freqView*, pan*) used densely throughout the zoom/pan/scrollbar code. Group
them into a Viewport sub-struct (app.view.*).

Rename anchored strictly to 'app.' so ScopeView's own view->viewStart/viewEnd
(same field names) are untouched. Settled render verified pixel-identical
(AE=0); End/zoom path verified via injected key input.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/spectrogram_types.h

@@ -106,6 +106,17 @@ typedef struct {
     float dragFreqStart;        // selection freq start when the move began
 } Selection;
 
+// The visible window into the spectrogram (time + frequency), all 0-1
+// normalized, plus the range captured at the start of a pan drag.
+typedef struct {
+    float start, end;                 // visible time range
+    float freqStart, freqEnd;         // visible freq range (0 = 0 Hz, 1 = Nyquist)
+    bool isPanning;
+    float panStart, panEnd;           // time range captured when the pan began
+    float panFreqStart, panFreqEnd;   // freq range captured when the pan began
+    Vector2 panStartPos;              // mouse pos when the pan began
+} Viewport;
+
 typedef struct {
     AudioSignal signal;
     StftResult stft;
@@ -126,17 +137,8 @@ typedef struct {
     char exportDir[4096];
     char exportMessage[256];
 
-    // Viewport/zoom controls
-    float viewStart;      // 0-1, start of visible time region
-    float viewEnd;        // 0-1, end of visible time region
-    float freqViewStart;  // 0-1, start of visible frequency region (0 = 0Hz)
-    float freqViewEnd;    // 0-1, end of visible frequency region (1 = Nyquist)
-    bool isPanning;
-    float panStartViewStart;
-    float panStartViewEnd;
-    float panStartFreqViewStart;
-    float panStartFreqViewEnd;
-    Vector2 panStartPos;
+    // Visible viewport (time + frequency) and in-progress pan state.
+    Viewport view;
 
     // Cached visible texture
     Texture2D visibleTexture;