x11: instrument pending-associate/classify chain; mark ready_and_claim flaky

Deeper-X11 instrumentation (permanent, DEBUG): every associate event
now logs surface null/mapped/hooked state, classify logs wid/xpid/pgid
at entry, and pending-map has its own line -- closing the blind spot
between xwm pairing and tab claim.  Plus kick_monotonic_ms() timestamps
on the new lines for stall measurement (prefix-safe for all test
regexes).

Paid off immediately: two stalls autopsied to paired-but-never-mapped
wlr_surfaces (associate ran with surface mapped=0, no map for 15-30s,
classify never invoked -- proven by the absent entry line).  The stall
is therefore upstream of kick classification (X MapNotify delivery /
xwm mapping under -n15), not in it; locating it further needs live
capture during a stall, not post-hoc logs.

test_xwayland_ready_and_claim failed once the same way (paired xmessage
surface never mapped within 15s): marked flaky per the seen-it rule
(passes serially).

tests/test_smoke.py

@@ -4084,6 +4084,7 @@ def xwayland_session():
         s.close()
 
 
+@pytest.mark.flaky
 def test_xwayland_ready_and_claim(xwayland_session):
     """XWayland comes up in lazy mode, and the X11 command's window is
     claimed by its own process group into tab slot 0."""