Re: BUG: Cascading standby fails to reconnect after falling back to archive recovery

Here are the v4 patches implementing what I described above.

On top of Xuneng's v3 (keeping the wait_for_event and scoped log
window test improvements), the main changes are:

- The wait is now capped at one wal_segment_size. If the gap is
larger, we skip the wait and let START_REPLICATION fail normally
so the startup process can fall back to archive. This avoids
indefinite polling when the upstream is fundamentally behind.

- The first "ahead of flush position" message is logged at LOG,
subsequent ones at DEBUG1, to cut down on noise during a long wait.

Two patches attached: v4-0001 for master (extends the
walrcv_identify_system API with an optional server_lsn output
parameter) and v4-backpatch-0001 for stable branches (uses a global
variable to preserve ABI, per Alvaro's suggestion).

Both pass the new TAP test.

Best regards,
Marco