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/power.h

@@ -1,10 +1,11 @@
 /**
  * @file power.h
- * @brief Power management: DC/DC regulator, GPIOTE wakeup, SYSTEM_ON sleep.
+ * @brief Power management: DC/DC regulator, GPIOTE wakeup, WFI sleep.
  */
 #pragma once
+#include <stdbool.h>
 
-/** @brief Enable the DC/DC converter and configure GPIOTE wakeup on the PTT button. */
+/** @brief Enable the DC/DC converter, configure GPIO input and GPIOTE wakeup on the PTT button. */
 void power_init(void);
 
 /**
@@ -14,3 +15,9 @@ void power_init(void);
  * interrupt fires (button press) and resumes from here.
  */
 void power_sleep_until_button(void);
+
+/**
+ * @brief Return the current state of the PTT button.
+ * @return true when the button is pressed.
+ */
+bool power_button_pressed(void);