Property-guarded trace logging for concurrent race-condition investigation.

Provides thread-safe stderr logging tagged with a global monotonic sequence
number and the current thread name. The sequence number gives an unambiguous
total order across threads, so the *last* events before a lost-wakeup
quiescence pinpoint the dropped notifier/terminator.

Activated only when -Dclj-r2dbc.trace=true is set; when disabled, `dlog` is a
no-op guarded by a single volatile read, with negligible JIT overhead.

To minimise the observer effect on the race under investigation, `dlog` does
NOT take a coarse application lock: it builds the whole line as one string and
emits it with a single `PrintStream.println` (internally atomic), so the only
added cross-thread synchronisation is the lock-free `AtomicLong` sequence. The
sequence number — not wall-clock ordering of interleaved writes — is the
authoritative event order.

This namespace is an implementation detail; do not use from application code.