fix(fpga): add FpgaResetBitstream() to force re-initialization

After aggressive field cycling (e.g. tear-off attacks), the FPGA's
internal SSC/DMA state can become corrupted even though the bitstream
is technically loaded. FpgaDownloadAndGo() caches downloaded_bitstream
and skips re-download if it matches, so subsequent commands fail
silently or hang.

Add FpgaResetBitstream() which sets downloaded_bitstream to
FPGA_BITSTREAM_UNKNOWN, forcing the next FpgaDownloadAndGo() to
perform a complete reload.

armsrc/fpgaloader.c

@@ -630,6 +630,10 @@ int FpgaGetCurrent(void) {
     return downloaded_bitstream;
 }
 
+void FpgaResetBitstream(void) {
+    downloaded_bitstream = FPGA_BITSTREAM_UNKNOWN;
+}
+
 // Turns off the antenna,
 // log message
 // if HF,  Disable SSC DMA

armsrc/fpgaloader.h

@@ -172,6 +172,7 @@ void SetupSpi(int mode);
 bool FpgaSetupSscDma(uint8_t *buf, uint16_t len);
 void Fpga_print_status(void);
 int FpgaGetCurrent(void);
+void FpgaResetBitstream(void);
 void SetAdcMuxFor(uint32_t whichGpio);
 
 // extern and generel turn off the antenna method