Implement PAwR response slot handling and HCI event generation

[Copilot]

Completes bidirectional PAwR communication by implementing response slot reception (advertiser) and transmission (scanner), plus HCI event reporting to host.

LLL Layer - Response Slot Handling

Advertiser (lll_adv_sync.c):

• After subevent TX, schedules response slot RX using response_slot_delay and response_slot_spacing
• isr_rx_response_slot() receives responses, validates CRC, extracts RSSI and data
• Processes multiple response slots iteratively, creates NODE_RX_TYPE_PAWR_RESPONSE for each

Scanner (lll_sync.c):

• After subevent RX, checks rsp_data.is_pending flag
• setup_response_slot_tx() calculates slot timing and schedules radio TX
• isr_tx_response_slot() transmits response PDU, clears pending flag

Timing: delay = response_slot_delay * 1.25ms + slot_number * response_slot_spacing * 0.125ms

ULL/HCI - Response Event Path

Node Processing:

• Added NODE_RX_TYPE_PAWR_RESPONSE case in ll_rx_dequeue()
• ull_adv_sync_pawr_response_rx() routes to HCI encoder

HCI Event (hci.c):

• le_per_adv_response_report() generates BT_HCI_EVT_LE_PER_ADV_RESPONSE_REPORT (0x3E/0x28)
• Includes: adv_handle, subevent, response_slot, RSSI, CRC status, data payload
• Respects event mask before transmission

Data Structure:

struct node_rx_pawr_response {
    struct node_rx_hdr hdr;
    struct node_rx_ftr rx_ftr;
    int8_t  rssi;
    uint8_t crc_ok:1;
    uint8_t subevent;
    uint8_t response_slot;
    uint8_t data_len;
    uint8_t data[];
};

Test Updates

Enhanced BabbleSim test to validate complete round-trip:

• Advertiser registers response callback, waits for and validates response data
• Scanner implements sync state machine, receives subevent data, queues and transmits response
• Verifies HCI event delivery and data integrity

Test validates: Parameter setting → subevent TX → subevent RX → response TX → response RX → HCI event → host callback

Data Flow

Advertiser → Subevent TX → [Radio] → Subevent RX → Scanner
Scanner → Response TX → [Radio] → Response RX → Advertiser
Advertiser → NODE_RX_TYPE_PAWR_RESPONSE → ULL → HCI Event 0x28 → Host

Current Support

✅ Single subevent operation
✅ Multiple response slots per subevent
✅ Response reception with RSSI/CRC
✅ HCI RESPONSE_REPORT event (0x28)
✅ Bidirectional data validation

⏸️ Multi-subevent scheduling (future)
⏸️ SUBEVENT_DATA_REQUEST event (0x27) (future)

Original prompt

Add support for Bluetooth Core Specification v5.4 Periodic Advertising with Responses feature in the open source Zephyr Bluetooth Controller implementation. Create the pull request in cvinayak/zephyr repository.

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