fix: MP3 recordings carried a hiss the WAV of the same audio did not

The operator split it in one test: WAV is fine, MP3 is not. The MP3 path is the
only one that resamples — 16 kHz up to 32 kHz, because the encoder emits broken
frames at MPEG-2 rates — and it did so by averaging neighbouring samples.

That is an upsampler in name only. Every component at f is mirrored to
16 kHz − f, and the test measures the mirror just 7 dB below the wanted signal:
a second copy of the whole spectrum. On speech it adds brightness; on receiver
noise, which is broadband, it lays a second noise floor over the top of the
band. Hence "a hiss that is not the QRM, louder than the voice" — and hence its
absence from the WAV, which resamples nothing.

Zero-stuffing plus a 63-tap windowed sinc at the old Nyquist puts the image
56.8 dB down instead of 7, at a cost measured in microseconds against the MP3
encode that follows. Two tests pin it: the image suppression, and that the level
is unchanged — a fix that quietly halved every recording would be a second
surprise on top of the one being repaired.

changelog.json

@@ -10,7 +10,7 @@
       "The \"From radio\" and mic levels take effect as you drag the slider, and the RX level now applies to what you HEAR through OpsLog, not only to recordings.",
       "The padlocks on frequency, band, mode and times no longer flicker under the pointer and refuse the click.",
       "The callsign field is wider, taking the room from the RST pair.",
-      "QSO recordings are no longer swamped by hiss: the audio was resampled with the cheap converter, folding everything above 8 kHz back onto the voice."
+      "MP3 recordings no longer carry a hiss of their own: the 16→32 kHz conversion mirrored the whole spectrum only 7 dB down. It is now 57 dB down."
     ],
     "fr": [
       "Les filtres du cluster gagnent NEW POTA et NEW COUNTY, à côté des pastilles de statut existantes.",
@@ -20,7 +20,7 @@
       "Les niveaux « From radio » et micro agissent pendant que vous déplacez le curseur, et le niveau RX s'applique désormais à ce que vous ENTENDEZ dans OpsLog, pas seulement aux enregistrements.",
       "Les cadenas de fréquence, bande, mode et heures ne scintillent plus sous le curseur et acceptent le clic.",
       "Le champ indicatif est plus large, la place venant de la paire RST.",
-      "Les enregistrements de QSO ne sont plus noyés par le souffle : l'audio était rééchantillonné avec le convertisseur économique, qui repliait tout ce qui dépasse 8 kHz sur la voix."
+      "Les enregistrements MP3 ne portent plus un souffle qui leur est propre : la conversion 16→32 kHz recopiait tout le spectre à seulement 7 dB en dessous. Il est désormais à 57 dB."
     ]
   },
   {