watcher: a lookup cut short by shutdown is not logged as an error (closes #229)

Stopping dnswatcher during a DNS check logged every lookup the stop cut
short as an error, "context canceled". The four places in the watcher
that log a failed lookup now do it through logFailedLookup, which logs
nothing when the watcher's context was cancelled; the port and TLS
checks already return quietly. It asks the context, not the lookup's
error, because the resolver reports a cancelled lookup with its own
error, which does not wrap context.Canceled. A context whose deadline
passed is not cancelled, so a lookup that ran out of time is still
logged as an error. The tests run a check on a cancelled context and on
one whose deadline passed; neither sends a query.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-02: a DNS lookup that shutdown cuts short logs no error; one that
+  fails otherwise, or runs out of time, still does (closes #229).
 - 2026-10-02: a resolver test that reads one record type from a nameserver's
   answer asks again when that type is missing from it (closes #218).
 - 2026-10-02: a plain `docker build .` of a clone stamps its tag or short