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/regs.h

@@ -153,3 +153,24 @@ typedef union {
     } bit;
     uint32_t reg;
 } radio_shorts_t;
+
+/* GPIO */
+
+/** @brief GPIO PIN_CNF[n]: pin configuration register. */
+typedef union {
+    struct {
+        uint32_t DIR   : 1;  /* [0]     0=Input  1=Output */
+        uint32_t INPUT : 1;  /* [1]     0=Connect  1=Disconnect */
+        uint32_t PULL  : 2;  /* [3:2]   0=Disabled  1=Pulldown  3=Pullup */
+        uint32_t       : 4;  /* [7:4]   reserved */
+        uint32_t DRIVE : 3;  /* [10:8]  0=S0S1 standard drive */
+        uint32_t       : 5;  /* [15:11] reserved */
+        uint32_t SENSE : 2;  /* [17:16] 0=Disabled  2=SenseHigh  3=SenseLow */
+        uint32_t       : 14; /* [31:18] reserved */
+    } bit;
+    uint32_t reg;
+} gpio_pin_cnf_t;
+
+#define GPIO_PULL_DISABLED 0u
+#define GPIO_PULL_PULLDOWN 1u
+#define GPIO_PULL_PULLUP 3u