Bluetooth: controller: Generate lost ISO data for CIS Rx

[Copilot]

The Nordic LLL advanced CIS Rx payload counters (nesn, payload_count) for unreceived payloads but emitted no ISO data for them. When a whole SDU's worth of payloads went missing, the ISO‑AL never advanced and never produced a lost SDU, leaving the Host with no gap indication and no way to drive LC3 Packet Loss Concealment (PLC).

This adds generation of HCI ISO data marked ISOAL_PDU_STATUS_LOST_DATA whenever a CIS Rx payload is skipped, for both central and peripheral roles.

Changes

• iso_rx_data_lost() helper (in lll_central_iso.c and lll_peripheral_iso.c): allocates an Rx node, sets the lost payload number, an approximate reference‑anchor timestamp, and status = ISOAL_PDU_STATUS_LOST_DATA, then enqueues it to the ISO‑AL. Gated on a configured Rx data path; uses a peek‑2/alloc‑1 guard so a buffer always remains for radio DMA. Mirrors the BIS (sync‑ISO) lost‑data path.
• Invocation at every Rx payload‑skip site:
  • payload_count_flush() — subevents not received within an event (flush timeout elapsed).
  • payload_count_flush_or_inc_on_close() / payload_count_rx_flush_or_txrx_inc() — payloads flushed when closing the isochronous event.
  • payload_count_lazy_update() / payload_count_lazy() — whole CIG events skipped due to overlapping events (laziness).

The ISO‑AL turns each lost PDU into a lost SDU (HCI Packet_Status_Flag 0b10), which the Host uses for LC3 PLC.

/* Generate HCI ISO data with lost status for an Rx payload that was
 * not received before its flush timeout / due to laziness.
 */
iso_rx_data_lost(cis_lll);

/* sn and nesn are 1-bit, only Least Significant bit is needed */
cis_lll->nesn++;
cis_lll->rx.bn_curr++;

Notes

• The payload number reuses the existing valid‑Rx convention (rx.payload_count + rx.bn_curr - 1U, bn_curr pre‑increment, 1‑based).
• The laziness path runs in prepare context before the current event's radio anchor is set, so its lost‑PDU timestamps are approximate; payload numbers and status are exact, and subsequent valid SDUs re‑anchor timing. Lost SDUs carry no audio, so PLC synthesizes them regardless.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bluetooth: Controller: Design and implementation of LLL CIS subevents (interleaved)</issue_title>
<issue_description>Is your enhancement proposal related to a problem? Please describe:

Sequential packing is implemented in the main branch in the Nordic LLL for Connected ISO.

Todo for this enhancement issue is to have, Connected ISO to support configurable parameter with interleaved scheduling; The HCI parameter is called "packing" where 0 - Sequential, 1- interleaved.

Describe the solution you'd like:

Update the Nordic LLL for connected ISO, both central and peripheral connected ISO roles to support interleaved packing. Implementation such that existing support for sequential packing can be separately conditionally compile-able. See how Broadcast ISO sequential and interleaved packing implementation is being independently conditionally compile-able.

Describe alternatives you've considered:

None for now.

Additional context:

Refer to BLUETOOTH CORE SPECIFICATION Version 6.2 | Vol 6, Part B, Section 4.5.14.2 Arrangement of multiple CISes

The CISes in a CIG shall be arranged either sequentially or interleaved by setting
the values of the Sub_Interval and the spacing between the CIS anchor points
appropriately.
If they are sequential, the CIS events of the different CISes do not overlap and so all the
subevents of a CIS event occur together. For each adjacent pair of CISes, the interval
between their CIS anchor points shall be at least NSE × Sub_Interval, using the values
for the lower numbered CIS.
If they are interleaved, all the first subevents of the CISes are adjacent, followed by the
second subevents, and so on. For each CIS, its value of Sub_Interval shall be at least
the sum of the values of SE_Length for all the CISes in the CIG. For each adjacent pair
of CISes, the interval between their CIS anchor points shall be at least SE_Length of
the lower numbered CIS.

Figure 4.67 shows each arrangement for a CIG with two CISes and NSE = 2.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Bluetooth: Controller: Design and implementation of LLL CIS subevents (interleaved) zephyrproject-rtos/zephyr#55261

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[cvinayak]

@copilot Analyse the lll_central_iso.c, lll_peripheral_iso.c and lll_conn_iso.c files for how sequential packing subevent are scheduled for radio transmission and reception. And add new implementation to support interleaved packing. Keep sequential packing under conditional compilation so that either or both of sequential and interleaved packing can be conditionally supported.

[Copilot]

@copilot Analyse the lll_central_iso.c, lll_peripheral_iso.c and lll_conn_iso.c files for how sequential packing subevent are scheduled for radio transmission and reception. And add new implemen...

I've analyzed the LLL layer files and documented how sequential and interleaved packing are supported.

Key Finding: Unlike BIS which requires separate channel calculation functions (next_chan_calc_seq vs next_chan_calc_int), the CIS LLL layer inherently supports both packing modes through a unified scheduling formula:

subevent_time = offset + (sub_interval * se_curr)

The ULL layer sets different offset and sub_interval values based on packing:

• Sequential: offset varies per CIS, sub_interval = SE_Length
• Interleaved: offset distributed evenly, sub_interval = num_cis × max_se_length

Changes made (commit 2a798c5):

• Added comprehensive comments in lll_central_iso.c explaining how both modes work
• Added comprehensive comments in lll_peripheral_iso.c explaining how both modes work
• Documented the unified scheduling approach that works for both packing modes

The LLL code doesn't need separate conditional compilation paths because the scheduling logic is generic and parameterized by ULL. The conditional compilation is already in place at the ULL layer where the packing-specific offset and sub_interval calculations occur.