UA1ZBE Custom Firmware v1.0

Features:
- VFO mode with direct frequency input
- POCSAG decoder (512/1200 baud) with BCH(31,21) correction
- Full-screen Spectrum analyzer
- FM Radio receiver
- RSSI signal indicator overlay
- Custom boot splash (UA1ZBE / POCSAG pager / build date)

Architecture:
- app/mode.c — mode dispatcher (VFO/POCSAG/Spectrum/FM)
- app/boot_splash.c — 2-second boot splash
- app/pocsag/ — POCSAG decoder + BCH correction
- app/display_rssi.c — RSSI indicator
- main.c — entry point with custom init
- syscalls.c — bare-metal _sbrk stub

Build: arm-none-eabi-gcc -Os -flto -Wall -Werror -Wextra
Size: 57.9KB Flash / 3.6KB RAM

Controls:
- 0-9: Direct frequency input (VFO)
- SK2: POCSAG mode
- SK1: Spectrum analyzer
- 0: FM Radio
- EXIT: Return to VFO
- F/*: Toggle 512/1200 baud (in POCSAG)

app/pocsag/bch31.h

@@ -0,0 +1,25 @@
+/* UA1ZBE Custom Firmware - BCH(31,21) header */
+#ifndef APP_POCSAG_BCH31_H
+#define APP_POCSAG_BCH31_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/* Correct errors in a POCSAG 32-bit word in place.
+ * Returns: 0 = no error, 1 = single corrected,
+ *          2 = double corrected, -1 = uncorrectable */
+int bch31_correct(uint32_t *word);
+
+/* Extract 21 data bits from corrected word */
+uint32_t bch31_get_data(uint32_t word);
+
+/* Get 2-bit function code */
+uint8_t bch31_get_func(uint32_t word);
+
+/* Check if word is sync word */
+bool bch31_is_sync(uint32_t word);
+
+/* Check if word is idle word */
+bool bch31_is_idle(uint32_t word);
+
+#endif