Remove DNS mocking from tests; use live DNS everywhere

DNS is never mocked in this repository: tests exercise live DNS,
and robustness comes from handling real-world DNS behavior with
tolerant assertions and sensible timeouts, not from mocks.

watcher: drop mockResolver and wire the real iterative resolver
into the tests, querying stable public names (example.com,
www.example.com). Change detection is exercised by seeding the
state store with a synthetic previous observation that live DNS
cannot match (reserved .invalid nameserver names and RFC 5737
documentation addresses); DNS stays live in every run. The port
checker, TLS checker, and notifier remain test doubles since they
are not DNS, keeping notification and state assertions
deterministic against whatever addresses live DNS returns.

resolver: drop the timeoutClient fake DNSClient and the
NewFromLoggerWithClient mock constructor. The timeout test is
replaced by a live query against an RFC 5737 documentation
address where no nameserver can exist, asserting a classified
non-OK response with no records.

TESTING.md: extend the live-DNS policy to every package and
remove the carve-out that permitted DNS mocks in packages that
consume the resolver.

TODO.md: update stale references to hermetic mocked-DNS work to
reflect the no-mocking policy and the current state of
feature/resolver.

Intentionally dropped coverage: the exact StatusTimeout
classification (previously forced by the fake client) is no
longer asserted, because a genuinely unreachable server may fail
fast instead of timing out depending on the network path; the
live test tolerantly accepts any failure classification.

TESTING.md

@@ -2,8 +2,10 @@
 
 ## DNS Resolution Tests
 
-All resolver tests **MUST** use live queries against real DNS servers.
-No mocking of the DNS client layer is permitted.
+All tests that involve DNS resolution — in every package, including
+consumers of the resolver such as the watcher — **MUST** use live
+queries against real DNS servers. No mocking, faking, or stubbing of
+DNS at any layer is permitted.
 
 ### Rationale
 
@@ -12,6 +14,8 @@ the full delegation chain. Mocked responses cannot faithfully represent
 the variety of real-world DNS behavior (truncation, referrals, glue
 records, DNSSEC, varied response times, EDNS, etc.). Testing against
 real servers ensures the resolver works correctly in production.
+Robustness comes from handling real-world DNS behavior with tolerant
+assertions and sensible timeouts, not from mocks.
 
 ### Constraints
 
@@ -24,11 +28,14 @@ real servers ensures the resolver works correctly in production.
 - Flaky failures from transient network issues are acceptable and
   should be investigated as potential resolver bugs, not papered over
   with mocks or skip flags
+- Watcher change-detection tests seed a synthetic *previous state*
+  and compare it against fresh live lookups; the DNS side is never
+  faked
 
 ### What NOT to do
 
-- **Do not mock `DNSClient`** for resolver tests (the mock constructor
-  exists for unit-testing other packages that consume the resolver)
+- **Do not mock `DNSClient`**, the watcher's `DNSResolver` interface,
+  or any other DNS abstraction — in any package, for any reason
 - **Do not add `-short` flags** to skip slow tests
 - **Do not increase `-timeout`** to hide hanging queries
 - **Do not modify linter configuration** to suppress findings