Implement full FHSS TX/RX, real AES key, button read, slot sync

radio_tx_burst(): captures current slot, calls fhss_next_channel() to
advance the sequence, sets channel, transmits ptt_frame_t, then holds
the dwell window via TIMER0 so the receiver has the full 2 ms to listen.

radio_rx_burst(): advances to the same channel, enables RX, waits for
EVENTS_END or TIMER0 COMPARE0 expiry. On a valid CRC the ptt_frame_t
is returned; caller syncs FHSS with fhss_set_slot(frame.slot + 1).

TIMER0: configured once in radio_init() (MODE=Timer, BITMODE=16-bit,
PRESCALER=4 -> 1 MHz tick, CC[0]=2000 -> 2 ms, COMPARE0_STOP shortcut).

fhss.c: replace zero key with a non-trivial 128-bit value; add
fhss_set_slot() and fhss_get_slot() for RX synchronisation.

power.c: add BUTTON_ACTIVE_LOW flag (default 1 = pull-up + GND button);
configure PIN_CNF accordingly; implement power_button_pressed() via
NRF_P0->IN.

regs.h: add gpio_pin_cnf_t with GPIO_PULL_* constants.

main.c: tight PTT loop -- radio_tx_burst() while button held, otherwise
radio_rx_burst() with slot sync on reception. No WFI in RX mode.

include/fhss.h

@@ -10,18 +10,29 @@
 #pragma once
 #include <stdint.h>
 
+/** Dwell time per channel in milliseconds. */
+#define FHSS_DWELL_MS 2u
+
+/** Number of channels in the hopping sequence. */
+#define FHSS_CHANNELS 40u
+
 /** @brief Reset the slot counter to zero. */
 void fhss_init(void);
 
 /**
- * @brief Return the next channel in the hopping sequence.
- *
- * Encrypts the current slot counter big-endian with AES-128-ECB, returns
- * @c block[0] % 40, and advances the slot counter.
- *
+ * @brief Return the next channel in the hopping sequence and advance the slot.
  * @return Channel index in [0, 39].
  */
 uint8_t fhss_next_channel(void);
 
-/** @brief Advance the slot counter without transmitting (receiver side). */
+/** @brief Advance the slot counter by one (receiver side, no packet received). */
 void fhss_sync_tick(void);
+
+/**
+ * @brief Force the slot counter to a specific value for RX synchronisation.
+ * @param s New slot value.
+ */
+void fhss_set_slot(uint32_t s);
+
+/** @brief Return the current slot counter value. */
+uint32_t fhss_get_slot(void);