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/render.c

@@ -283,7 +283,7 @@ void DrawLabels(Rectangle bounds)
     // Time labels
     for (int i = 0; i <= 10; i++) {
         float t = (float)i / 10;
-        float timeSec = (app.viewStart + t * (app.viewEnd - app.viewStart)) * app.signal.duration;
+        float timeSec = (app.view.start + t * (app.view.end - app.view.start)) * app.signal.duration;
         float x = bounds.x + t * bounds.width;
         char label[32];
         if (timeSec >= 60) sprintf(label, "%d:%02d", (int)(timeSec / 60), (int)(timeSec) % 60);
@@ -293,8 +293,8 @@ void DrawLabels(Rectangle bounds)
 
     // Frequency labels adapted to current zoom level
     float maxFreq = (float)app.signal.sampleRate / 2.0f;
-    float freqMin = app.freqViewStart * maxFreq;
-    float freqMax = app.freqViewEnd * maxFreq;
+    float freqMin = app.view.freqStart * maxFreq;
+    float freqMax = app.view.freqEnd * maxFreq;
 
     // Choose tick spacing based on zoom range
     float freqRange = freqMax - freqMin;
@@ -348,13 +348,13 @@ void DrawSelection(Rectangle bounds)
     Color overlayColor = Fade(BLACK, 0.25f);  // Lighter overlay
     
     // Convert signal coordinates to viewport coordinates
-    float viewWidth = app.viewEnd - app.viewStart;
-    float freqWidth = app.freqViewEnd - app.freqViewStart;
+    float viewWidth = app.view.end - app.view.start;
+    float freqWidth = app.view.freqEnd - app.view.freqStart;
     
-    float selStartX = bounds.x + ((app.sel.timeStart - app.viewStart) / viewWidth) * bounds.width;
-    float selEndX = bounds.x + ((app.sel.timeEnd - app.viewStart) / viewWidth) * bounds.width;
-    float selStartY = bounds.y + bounds.height - ((app.sel.freqEnd - app.freqViewStart) / freqWidth) * bounds.height;
-    float selEndY = bounds.y + bounds.height - ((app.sel.freqStart - app.freqViewStart) / freqWidth) * bounds.height;
+    float selStartX = bounds.x + ((app.sel.timeStart - app.view.start) / viewWidth) * bounds.width;
+    float selEndX = bounds.x + ((app.sel.timeEnd - app.view.start) / viewWidth) * bounds.width;
+    float selStartY = bounds.y + bounds.height - ((app.sel.freqEnd - app.view.freqStart) / freqWidth) * bounds.height;
+    float selEndY = bounds.y + bounds.height - ((app.sel.freqStart - app.view.freqStart) / freqWidth) * bounds.height;
     
     // Clamp to viewport bounds
     selStartX = fmaxf(bounds.x, fminf(bounds.x + bounds.width, selStartX));
@@ -443,13 +443,13 @@ void DrawSelectionDrag(Rectangle bounds)
         (app.sel.isDragging && !IsMouseButtonDown(MOUSE_LEFT_BUTTON))) return;
     
     // Convert signal coordinates to viewport coordinates
-    float viewWidth = app.viewEnd - app.viewStart;
-    float freqWidth = app.freqViewEnd - app.freqViewStart;
+    float viewWidth = app.view.end - app.view.start;
+    float freqWidth = app.view.freqEnd - app.view.freqStart;
     
-    float selStartX = bounds.x + ((app.sel.timeStart - app.viewStart) / viewWidth) * bounds.width;
-    float selEndX = bounds.x + ((app.sel.timeEnd - app.viewStart) / viewWidth) * bounds.width;
-    float selStartY = bounds.y + bounds.height - ((app.sel.freqEnd - app.freqViewStart) / freqWidth) * bounds.height;
-    float selEndY = bounds.y + bounds.height - ((app.sel.freqStart - app.freqViewStart) / freqWidth) * bounds.height;
+    float selStartX = bounds.x + ((app.sel.timeStart - app.view.start) / viewWidth) * bounds.width;
+    float selEndX = bounds.x + ((app.sel.timeEnd - app.view.start) / viewWidth) * bounds.width;
+    float selStartY = bounds.y + bounds.height - ((app.sel.freqEnd - app.view.freqStart) / freqWidth) * bounds.height;
+    float selEndY = bounds.y + bounds.height - ((app.sel.freqStart - app.view.freqStart) / freqWidth) * bounds.height;
     
     // Clamp to viewport bounds
     selStartX = fmaxf(bounds.x, fminf(bounds.x + bounds.width, selStartX));
@@ -539,8 +539,8 @@ void DrawPlayhead(Rectangle bounds)
     if (!app.isPlaying || app.playheadT < 0.0f || app.playheadT > 1.0f) return;
 
     float timePos = app.sel.timeStart + app.playheadT * (app.sel.timeEnd - app.sel.timeStart);
-    float viewWidth = app.viewEnd - app.viewStart;
-    float t = (timePos - app.viewStart) / viewWidth;
+    float viewWidth = app.view.end - app.view.start;
+    float t = (timePos - app.view.start) / viewWidth;
     float x = bounds.x + t * bounds.width;
 
     // Clamp to bounds