Pins down the public-API behavior of AsyncLogWriter so future tightening
of the PII redaction regexes (#2478) cannot regress without a test
failing. Cases mirror the suggested set in the issue:

- Line shape preservation ([HH:mm:ss.zzz] LVL cat: msg\n) for every
  QtMsgType label
- IPv4 redaction, ver= exemption, and lock-in that quoted four-octet
  IPs are still redacted (per triage: quoting is not authentication of
  intent). The original "0.9.8" case was dropped as non-applicable
  (three octets never match the regex) and replaced with the explicit
  three-octet pass-through assertion.
- Radio serial, id_token, MAC dash, and MAC colon redaction
- clearLog truncates pre-clear lines and preserves post-clear lines
- shutdown drains the queue (no loss on graceful stop)
- Drop accounting under burst load (asserts inequalities since the
  worker drains concurrently; per triage open-question #2)
- High-priority reserve keeps a critical line alive under a debug
  burst, and droppedHighPriorityLines stays at 0
- Stderr mirroring matches the file contents byte-for-byte when
  mirrorToStderr=true

Test uses only the public API (no test-only hooks added to
AsyncLogWriter), and routes the log path through QTemporaryDir so the
suite does not litter /tmp.

Blast radius: risk_score=0.000, 0 high-risk affected (AsyncLogWriter
itself has self_betweenness=0; pure additive test + CMakeLists entry).