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/radio.h

@@ -3,9 +3,23 @@
  * @brief RADIO peripheral driver -- NRF_1Mbit proprietary mode.
  */
 #pragma once
+#include <stdbool.h>
 #include <stdint.h>
 
-/** @brief Configure the RADIO peripheral (mode, packet format, address, CRC, power, channel). */
+/**
+ * @brief PTT packet transmitted on every FHSS hop.
+ *
+ * The receiver uses @p slot to resynchronise its FHSS counter after
+ * receiving the first packet.
+ */
+typedef struct __attribute__((packed)) {
+    uint32_t slot; /**< Sender's FHSS slot number at time of transmission. */
+    uint8_t flags; /**< Bitmask: PTT_FLAG_ACTIVE when voice channel is open. */
+} ptt_frame_t;
+
+#define PTT_FLAG_ACTIVE 0x01u /**< PTT button is held on the transmitting side. */
+
+/** @brief Configure the RADIO peripheral (mode, packet format, address, CRC, power). */
 void radio_init(void);
 
 /**
@@ -18,13 +32,30 @@ void radio_set_channel(uint8_t ch);
  * @brief Transmit one packet synchronously.
  *
  * Loads @p data into the internal packet buffer, asserts TASKS_TXEN, and
- * returns after EVENTS_END fires.  The RADIO is DISABLED automatically via
- * the END_DISABLE shortcut before the function returns.
+ * returns after EVENTS_END fires.  RADIO is DISABLED automatically via the
+ * END_DISABLE shortcut before the function returns.
  *
  * @param data  Payload bytes.
  * @param len   Payload length (0-255 bytes).
  */
 void radio_tx(const uint8_t *data, uint8_t len);
 
-/** @brief Transmit a burst with FHSS hopping (not yet implemented). */
+/**
+ * @brief Transmit one FHSS hop: advance channel, send PTT frame, hold dwell time.
+ *
+ * Call repeatedly in a loop while the PTT button is held.  Each call occupies
+ * exactly FHSS_DWELL_MS milliseconds.
+ */
 void radio_tx_burst(void);
+
+/**
+ * @brief Receive one FHSS hop: advance channel, listen for FHSS_DWELL_MS ms.
+ *
+ * If a packet with a valid CRC arrives during the dwell window, @p frame_out
+ * is filled and the function returns true.  The caller should then call
+ * fhss_set_slot(frame_out->slot + 1) to synchronise the hopping sequence.
+ *
+ * @param frame_out  Destination for the received frame (must not be NULL).
+ * @return true if a valid packet was received, false on timeout or CRC error.
+ */
+bool radio_rx_burst(ptt_frame_t *frame_out);