Add AM spectrum ZOOM x3/x4 with optimized x4 decimation (#3304)

* Add AM spectrum zoom x3 and x4
* Optimize AM spectrum zoom x4 processing
* Fix AM spectrum zoom formatting
* Fix AM spectrum zoom header formatting

firmware/common/dsp_fir_taps.hpp

@@ -77,6 +77,8 @@ constexpr fir_taps_real<16> taps_audio_wide_halfband_0{
  * contiguous 256-sample FFT frame is being collected:
  *   384kHz -> 192kHz (Zoom x1)
  *   192kHz ->  96kHz (additional stage for Zoom x2)
+ *    96kHz ->  48kHz (additional stage for Zoom x3)
+ *    96kHz ->  48kHz -> 24kHz (after the direct /4 stage for Zoom x4)
  */
 constexpr fir_taps_real<63> taps_audio_spectrum_halfband{
     .low_frequency_normalized = -0.23f,
@@ -149,6 +151,49 @@ constexpr fir_taps_real<63> taps_audio_spectrum_halfband{
     }},
 };
 
+/* Direct 384kHz -> 96kHz spectrum decimator for AM Zoom x4.
+ * Equiripple low-pass: passband edge 25.92kHz, stopband edge 70.08kHz.
+ */
+constexpr fir_taps_real<32> taps_audio_spectrum_decim_4{
+    .low_frequency_normalized = -25920.0f / 384000.0f,
+    .high_frequency_normalized = 25920.0f / 384000.0f,
+    .transition_normalized = 44160.0f / 384000.0f,
+    .taps = {{
+        -26,
+        -62,
+        -79,
+        -24,
+        126,
+        312,
+        377,
+        154,
+        -389,
+        -1020,
+        -1263,
+        -616,
+        1137,
+        3685,
+        6234,
+        7829,
+        7829,
+        6234,
+        3685,
+        1137,
+        -616,
+        -1263,
+        -1020,
+        -389,
+        154,
+        377,
+        312,
+        126,
+        -24,
+        -79,
+        -62,
+        -26,
+    }},
+};
+
 // NBFM 16K0F3E emission type /////////////////////////////////////////////
 
 // IFIR image-reject filter: fs=3072000, pass=8000, stop=344000, decim=8, fout=384000

firmware/common/message.hpp

@@ -728,6 +728,8 @@ class AMConfigureMessage : public Message {
     enum class Zoom_waterfall : size_t {
         ZOOM_x_1 = 1,
         ZOOM_x_2 = 2,
+        ZOOM_x_3 = 4,
+        ZOOM_x_4 = 8,
     };
 
     constexpr AMConfigureMessage(