Implement basic radio TX on fixed channel

radio_init() configures RADIO in NRF_1Mbit proprietary mode: 8-bit
length field, 4-byte address (BASE0+PREFIX0), 2-byte CRC-16/CCITT,
0 dBm TX power, channel 20 (2420 MHz). READY->START and END->DISABLE
shortcuts let radio_tx() trigger the full ramp-up/TX/disable sequence
by writing TASKS_TXEN once, then polling EVENTS_END.

main.c sends a fixed 4-byte test frame on every button press.
FHSS and AES remain compiled but are not called.

New bitfield unions in regs.h: radio_pcnf0_t, radio_pcnf1_t,
radio_crccnf_t, radio_shorts_t.

include/regs.h

@@ -88,3 +88,67 @@ typedef union {
 #define RADIO_MODE_NRF_1MBIT  0u
 #define RADIO_MODE_NRF_2MBIT  1u
 #define RADIO_MODE_BLE_1MBIT  4u
+
+/* PCNF0: packet configuration register 0 */
+typedef union {
+    struct {
+        uint32_t LFLEN   : 4;   /* [3:0]   length of LENGTH field in bits */
+        uint32_t         : 4;   /* [7:4]   reserved */
+        uint32_t S0LEN   : 1;   /* [8]     length of S0 field in bytes (0 or 1) */
+        uint32_t         : 7;   /* [15:9]  reserved */
+        uint32_t S1LEN   : 4;   /* [19:16] length of S1 field in bits */
+        uint32_t S1INCL  : 1;   /* [20]    include S1 field in RAM even if zero length */
+        uint32_t         : 3;   /* [23:21] reserved */
+        uint32_t PLEN    : 1;   /* [24]    0=8-bit preamble  1=16-bit preamble */
+        uint32_t         : 6;   /* [30:25] reserved */
+        uint32_t CRCINC  : 1;   /* [31]    include CRC in LENGTH field */
+    } bit;
+    uint32_t reg;
+} radio_pcnf0_t;
+
+/* PCNF1: packet configuration register 1 */
+typedef union {
+    struct {
+        uint32_t MAXLEN  : 8;   /* [7:0]   maximum payload length in bytes */
+        uint32_t STATLEN : 8;   /* [15:8]  static length added to payload */
+        uint32_t BALEN   : 3;   /* [18:16] base address length (2-4 bytes) */
+        uint32_t         : 5;   /* [23:19] reserved */
+        uint32_t ENDIAN  : 1;   /* [24]    0=little-endian  1=big-endian */
+        uint32_t WHITEEN : 1;   /* [25]    1=enable data whitening */
+        uint32_t         : 6;   /* [31:26] reserved */
+    } bit;
+    uint32_t reg;
+} radio_pcnf1_t;
+
+/* CRCCNF: CRC configuration */
+typedef union {
+    struct {
+        uint32_t LEN      : 2;   /* [1:0]  0=disabled  1=1 byte  2=2 bytes  3=3 bytes */
+        uint32_t          : 6;   /* [7:2]  reserved */
+        uint32_t SKIPADDR : 1;   /* [8]    1=skip address field in CRC calculation */
+        uint32_t          : 23;  /* [31:9] reserved */
+    } bit;
+    uint32_t reg;
+} radio_crccnf_t;
+
+#define RADIO_CRCCNF_LEN_DISABLED  0u
+#define RADIO_CRCCNF_LEN_ONE       1u
+#define RADIO_CRCCNF_LEN_TWO       2u
+#define RADIO_CRCCNF_LEN_THREE     3u
+
+/* SHORTS: shortcut register */
+typedef union {
+    struct {
+        uint32_t READY_START        : 1;   /* [0]  READY -> TASKS_START */
+        uint32_t END_DISABLE        : 1;   /* [1]  END   -> TASKS_DISABLE */
+        uint32_t DISABLED_TXEN      : 1;   /* [2]  DISABLED -> TASKS_TXEN */
+        uint32_t DISABLED_RXEN      : 1;   /* [3]  DISABLED -> TASKS_RXEN */
+        uint32_t ADDRESS_RSSISTART  : 1;   /* [4]  ADDRESS  -> TASKS_RSSISTART */
+        uint32_t END_START          : 1;   /* [5]  END      -> TASKS_START */
+        uint32_t ADDRESS_BCSTART    : 1;   /* [6]  ADDRESS  -> TASKS_BCSTART */
+        uint32_t                    : 1;   /* [7]  reserved */
+        uint32_t DISABLED_RSSISTOP  : 1;   /* [8]  DISABLED -> TASKS_RSSISTOP */
+        uint32_t                    : 23;  /* [31:9] reserved */
+    } bit;
+    uint32_t reg;
+} radio_shorts_t;