refactor: group selection state into a Selection sub-struct + ClearSelection()

The box-selection state was 11 fields spread across three comment blocks
(time sel, freq sel, drag state). Consolidate them into one Selection
sub-struct (app.sel.*) and extract the 4-line 'clear to full range' block —
duplicated in 5 places — into a ClearSelection() helper.

Pure mechanical rename (anchored to app./spa-> so the ScopeView fields of the
same name are untouched) + behavior-identical dedup. Fully-settled render
verified pixel-identical (AE=0).

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

src/spectrogram_types.h

@@ -91,6 +91,21 @@ typedef struct {
     int nextOrder;
 } FFTSizeCache;
 
+// The time+frequency box selection and its drag/move interaction state.
+// All coordinates are 0-1 normalized. A "box-select" drags out a new box;
+// a "move" drags an existing box around.
+typedef struct {
+    float timeStart, timeEnd;   // selected time span
+    float freqStart, freqEnd;   // selected frequency span
+    bool isTimeSelecting;       // dragging out a new time span
+    bool isFreqSelecting;       // dragging out a new frequency span
+    Vector2 selectStartPos;     // mouse pos when a box-select began (min-drag check)
+    bool isDragging;            // moving an existing selection box
+    Vector2 dragStartPos;       // mouse pos when the move began
+    float dragTimeStart;        // selection start time when the move began
+    float dragFreqStart;        // selection freq start when the move began
+} Selection;
+
 typedef struct {
     AudioSignal signal;
     StftResult stft;
@@ -103,27 +118,14 @@ typedef struct {
     float playheadT;           // 0-1 normalized position in selection
     float playheadElapsed;     // Elapsed seconds since play started
 
-    // Time selection (0-1 normalized)
-    float timeSelectionStart;
-    float timeSelectionEnd;
-    bool isTimeSelecting;
-
-    // Frequency selection (0-1 normalized)
-    float freqSelectionStart;
-    float freqSelectionEnd;
-    bool isFreqSelecting;
+    // Time + frequency box selection and its drag/move interaction state.
+    Selection sel;
 
     // Export settings
     float exportScale;
     char exportDir[4096];
     char exportMessage[256];
 
-    Vector2 selectStartPos;  // For minimum drag distance check
-    bool isDraggingSelection;  // Dragging existing selection box
-    Vector2 dragSelectionStartPos;  // Mouse position when started dragging selection
-    float dragSelectionTimeStart;  // Selection start time when dragging
-    float dragSelectionFreqStart;  // Selection freq start when dragging
-
     // Viewport/zoom controls
     float viewStart;      // 0-1, start of visible time region
     float viewEnd;        // 0-1, end of visible time region
@@ -233,6 +235,13 @@ static inline bool UiModalOpen(void)
     return app.showFileBrowser || app.showAbout;
 }
 
+// Reset the box selection to the full signal (the "no selection" state).
+static inline void ClearSelection(void)
+{
+    app.sel.timeStart = 0.0f; app.sel.timeEnd = 1.0f;
+    app.sel.freqStart = 0.0f; app.sel.freqEnd = 1.0f;
+}
+
 // ============================================================================
 // Keymap — single source of truth for global key bindings.
 // The dispatcher (DispatchKeymap in spectrogram.c) runs every entry whose