feat: live cursor readout (time / frequency / STFT level)

Floating tag follows the pointer over the spectrogram showing the time,
frequency, and STFT magnitude (dB) under the cursor — the standard
spectrum-analyzer probe. Suppressed while selecting/panning (which have
their own readout) and when a modal is open.

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

src/render.c

@@ -481,6 +481,55 @@ void DrawSelectionDrag(Rectangle bounds)
     DrawStatPanel(bounds, (Rectangle){ x, y, w, h });
 }
 
+// Floating time/frequency/level tag that follows the cursor over the
+// spectrogram (suppressed while selecting/panning, which have their own
+// readout). The level is the STFT magnitude at that bin, in dB.
+void DrawCursorReadout(Rectangle bounds)
+{
+    if (!app.loaded || !app.stftComputed || app.signal.samples == NULL) return;
+    if (app.sel.isTimeSelecting || app.sel.isFreqSelecting || app.sel.isDragging ||
+        app.view.isPanning) return;
+    Vector2 m = GetMousePosition();
+    if (!CheckCollisionPointRec(m, bounds)) return;
+
+    float tFrac = app.view.start + ((m.x - bounds.x) / bounds.width) * (app.view.end - app.view.start);
+    float fFrac = app.view.freqStart + (1.0f - (m.y - bounds.y) / bounds.height) * (app.view.freqEnd - app.view.freqStart);
+    float timeSec = tFrac * app.signal.duration;
+    float nyquist = app.signal.sampleRate * 0.5f;
+    float freqHz = fFrac * nyquist;
+
+    // Sample the STFT level at this (time, freq).
+    char level[32] = "--";
+    if (app.stft.numSegments > 0) {
+        int seg = (int)(tFrac * app.stft.numSegments);
+        if (seg < 0) seg = 0;
+        if (seg >= app.stft.numSegments) seg = app.stft.numSegments - 1;
+        const StftSegment* s = &app.stft.segments[seg];
+        if (s->spectrum && s->numBins > 1) {
+            float binHz = nyquist / (float)(s->numBins - 1);
+            int bin = (int)(freqHz / binHz + 0.5f);
+            if (bin < 0) bin = 0;
+            if (bin >= s->numBins) bin = s->numBins - 1;
+            sprintf(level, "%.1f dB", AmplitudeToDecibels(s->spectrum[bin].amplitude));
+        }
+    }
+
+    char text[80];
+    sprintf(text, "%.3fs  %.0f Hz  %s", timeSec, freqHz, level);
+    int fontSize = 10;
+    int tw = MeasureText(text, fontSize);
+    int boxW = tw + 12, boxH = fontSize + 8;
+
+    // Offset up-right of the cursor; flip to keep it inside the viewport.
+    float bx = m.x + 14, by = m.y - boxH - 6;
+    if (bx + boxW > bounds.x + bounds.width) bx = m.x - boxW - 14;
+    if (by < bounds.y) by = m.y + 14;
+
+    DrawRectangle((int)bx, (int)by, boxW, boxH, (Color){ 0, 0, 0, 200 });
+    DrawRectangleLines((int)bx, (int)by, boxW, boxH, Fade(SKYBLUE, 0.6f));
+    DrawText(text, (int)bx + 6, (int)by + 4, fontSize, (Color){ 180, 220, 255, 255 });
+}
+
 // ============================================================================
 // Playhead
 // ============================================================================