Add an event-loop responsiveness watchdog (discussion #644 follow-up)

QetLogger (discussion #644, steps 1-3) captures whatever an explicit
qDebug()/qInfo()/qWarning() call already decided to report. Most of a
session -- painting, dragging, a slow synchronous operation -- produces
no log output at all, so a silent multi-second gap in the log is
indistinguishable from the user simply not doing anything. That gap
came up directly: investigating a user-reported "the program lagged"
required inferring stalls from timestamp gaps between unrelated log
lines, which can't tell a real freeze apart from normal idle time.

EventLoopWatchdog closes that gap directly instead of inferring it. A
QTimer::PreciseTimer repeating tick (every 50ms) measures the *actual*
elapsed time since the previous tick via QElapsedTimer (monotonic,
unaffected by system clock/NTP adjustments). Qt does not queue up
missed fires for a normal repeating timer, so if the main thread is
blocked for 600ms, the timer fires once as soon as the loop frees up,
with ~600ms measured since the last tick -- that gap is the stall,
measured at its source. Only logs (via the existing qWarning() path,
so it reuses QetLogger's file/ring/rotation with no new plumbing) when
a tick is late by more than 200ms, so a healthy session produces zero
output from this class, in keeping with QetLogger's bounded-log design.

Same QET_WATCHDOG_DISABLE=1 escape-hatch convention as QetLogger's own
QET_LOG_DISABLE=1.

Deliberately not included: attributing a stall to what caused it. This
tells you a stall happened and how long -- pairing that timestamp with
gdb attached to a running session (as used for the CLI hang, PR #661)
is still how you get from "it stalled" to a root cause.

Stacked on #647 (feature-diagnostic-logging-crash) for QetLogger/
qWarning() plumbing this depends on -- diff includes its commits until
that merges.

Verified against the compiled binary, not just read: temporarily
injected a QThread::msleep(600) via a one-shot QTimer 2s after
startup, confirmed the exact expected warning
("EventLoopWatchdog: main thread stalled for 620 ms") at the right
severity through the real qWarning()/QetLogger path, then removed the
test hook and reconfirmed a normal run produces no output from this
class at all.

cmake/qet_compilation_vars.cmake

@@ -118,6 +118,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/cli_export.h
   ${QET_DIR}/sources/logging/crashhandler.cpp
   ${QET_DIR}/sources/logging/crashhandler.h
+  ${QET_DIR}/sources/logging/eventloopwatchdog.cpp
+  ${QET_DIR}/sources/logging/eventloopwatchdog.h
   ${QET_DIR}/sources/logging/logring.cpp
   ${QET_DIR}/sources/logging/logring.h
   ${QET_DIR}/sources/logging/qetlogger.cpp