fix: align playback and WAV export with the displayed frequency axis

Follows 81bab18, which fixed the same class of bug in the PNG export path.
sel.freq* are fractions of the *displayed* frequency axis (capped at
EffectiveMaxFreqHz), not of true Nyquist, but BuildSelectionAudio and
ExportSelectionWAV both scaled them against sampleRate/2. With a display
crop active this widened the bandpass by 1/DisplayFreqFraction(), so the
audio you heard (and the WAV you exported) covered a higher, wider band
than the box you drew.

Convert through EffectiveMaxFreqHz for the selection bounds; the filter
still reasons in true-Nyquist terms, which is what the separate `nyquist`
local is for. The "is this effectively full-band?" test likewise compares
against nyquist, so an uncropped selection still skips filtering.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8ZWfr5XZyyDttvkhJUgHN

src/audio.c

@@ -204,10 +204,16 @@ static float* BuildSelectionAudio(int* outNumSamples)
     if (!regionSamples) return NULL;
     memcpy(regionSamples, app.signal.samples + startSample, numSamples * sizeof(float));
 
-    float maxFreq = (float)app.signal.sampleRate / 2.0f;
+    // sel.freq* are fractions of the *displayed* axis (capped at
+    // EffectiveMaxFreqHz), not of true Nyquist — same convention the PNG export
+    // in ui.c uses. Convert through EffectiveMaxFreqHz so a display crop doesn't
+    // scale the passband up by 1/DisplayFreqFraction(). The filter itself still
+    // works in true-Nyquist terms, which is what nyquist is for.
+    float nyquist = (float)app.signal.sampleRate / 2.0f;
+    float maxFreq = EffectiveMaxFreqHz();
     float freqLow = app.sel.freqStart * maxFreq;
     float freqHigh = app.sel.freqEnd * maxFreq;
-    if (freqLow > 10.0f || freqHigh < maxFreq - 10.0f) {
+    if (freqLow > 10.0f || freqHigh < nyquist - 10.0f) {
         TraceLog(LOG_INFO, "Applying bandpass filter: %.0f - %.0f Hz", freqLow, freqHigh);
         ApplyBandpassFilter(regionSamples, numSamples, app.signal.sampleRate, freqLow, freqHigh);
     }
@@ -257,7 +263,7 @@ void ExportSelectionWAV(const char* dirPath)
         return;
     }
 
-    float maxFreq = (float)app.signal.sampleRate / 2.0f;
+    float maxFreq = EffectiveMaxFreqHz();  // sel.freq* are display-axis fractions
     float t0 = app.sel.timeStart * app.signal.duration;
     float t1 = app.sel.timeEnd * app.signal.duration;
     float f0 = app.sel.freqStart * maxFreq;