feat: export selection to WAV (band-limited, time-cropped)

New "Export WAV (W)" button + W key saves the current selection as a mono
WAV — the same band-limited, time-cropped audio you'd hear on playback, so
"what you hear is what you save". Self-documenting filename encodes the
time span and frequency band (rspektrum_sel_<t0>-<t1>s_<f0>-<f1>Hz.wav).

Refactor the shared filtered-region builder out of PlaySelectedRegion
(which also fixes a leak: it never freed regionSamples after
LoadSoundFromWave copies it). Make the export confirmation message persist
~3s instead of flashing for a single frame (affected PNG export too).

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

src/spectrogram.c

@@ -154,6 +154,7 @@ static void ActionToggleScope(void)    { app.showScope = !app.showScope; }
 static void ActionToggleAbout(void)    { app.showAbout = !app.showAbout; }
 static void ActionToggleFullscreen(void){ ToggleFullscreen(); }
 static void ActionExport(void)         { ExportPNG(&app, app.exportDir); }
+static void ActionExportWav(void)      { ExportSelectionWAV(app.exportDir); }
 
 static void ActionResetView(void)
 {
@@ -177,6 +178,7 @@ static const KeyBinding KEYMAP[] = {
     { KEY_HOME,  KEYGATE_MODAL | KEYGATE_LOADED,ActionResetView,       "Home",  "reset view (fit all)" },
     { KEY_END,   KEYGATE_MODAL | KEYGATE_LOADED,ActionZoomToStart,     "End",   "zoom to start" },
     { KEY_E,     KEYGATE_MODAL | KEYGATE_STFT,  ActionExport,          "E",     "export PNG" },
+    { KEY_W,     KEYGATE_MODAL | KEYGATE_STFT,  ActionExportWav,       "W",     "export selection WAV" },
     // Order-sensitive: handled inline (see main loop), listed here for the overlay.
     { KEY_SPACE, KEYGATE_NONE,                  NULL,                  "Space", "play / stop selection" },
     { KEY_ESCAPE,KEYGATE_NONE,                  NULL,                  "Esc",   "clear selection / close dialog" },
@@ -1062,8 +1064,11 @@ int main(int argc, char* argv[])
             DrawText(app.exportMessage, boxX + (boxW - msgW) / 2, boxY + 10, 20, WHITE);
         }
 
-        // Reset export message after one frame display
-        app.exportMessage[0] = '\0';
+        // Hold the export message for a few seconds, then clear it.
+        if (app.exportMessageTimer > 0.0f) {
+            app.exportMessageTimer -= GetFrameTime();
+            if (app.exportMessageTimer <= 0.0f) app.exportMessage[0] = '\0';
+        }
 
         EndDrawing();
     }