fix(server): correct RSSI byte offset in frame parser (#332)

The server parsed rssi from buf[14] and noise_floor from buf[15], but
the firmware (csi_collector.c) packs them at buf[16] and buf[17]:

  Firmware:  n_subcarriers=u16(6-7) freq=u32(8-11) seq=u32(12-15) rssi=i8(16)
  Server:    n_subcarriers=u8(6)    freq=u16(8-9)  seq=u32(10-13) rssi=i8(14) ← WRONG

This caused RSSI to read the high byte of the sequence counter instead
of the actual signed RSSI value, producing positive values (e.g., +9)
instead of the correct negative values (e.g., -46 dBm).

Added inline documentation of the frame layout matching csi_collector.c.

Closes #332

rust-port/wifi-densepose-rs/crates/wifi-densepose-sensing-server/src/main.rs

@@ -565,13 +565,25 @@ fn parse_esp32_frame(buf: &[u8]) -> Option<Esp32Frame> {
         return None;
     }
 
+    // Frame layout (must match firmware csi_collector.c):
+    //   [0..3]   magic (u32 LE)
+    //   [4]      node_id (u8)
+    //   [5]      n_antennas (u8)
+    //   [6..7]   n_subcarriers (u16 LE)
+    //   [8..11]  freq_mhz (u32 LE)
+    //   [12..15] sequence (u32 LE)
+    //   [16]     rssi (i8)
+    //   [17]     noise_floor (i8)
+    //   [18..19] reserved
+    //   [20..]   I/Q data
     let node_id = buf[4];
     let n_antennas = buf[5];
-    let n_subcarriers = buf[6];
-    let freq_mhz = u16::from_le_bytes([buf[8], buf[9]]);
-    let sequence = u32::from_le_bytes([buf[10], buf[11], buf[12], buf[13]]);
-    let rssi = buf[14] as i8;
-    let noise_floor = buf[15] as i8;
+    let n_subcarriers_u16 = u16::from_le_bytes([buf[6], buf[7]]);
+    let n_subcarriers = n_subcarriers_u16 as u8; // truncate to u8 for Esp32Frame compat
+    let freq_mhz = u16::from_le_bytes([buf[8], buf[9]]); // low 16 bits of u32
+    let sequence = u32::from_le_bytes([buf[12], buf[13], buf[14], buf[15]]);
+    let rssi = buf[16] as i8;       // #332: was buf[14], 2 bytes off
+    let noise_floor = buf[17] as i8; // #332: was buf[15], 2 bytes off
 
     let iq_start = 20;
     let n_pairs = n_antennas as usize * n_subcarriers as usize;