QFw: let the logs tier carry the client's story lines

[DougSO]

What

The QFw Trace page's "Log lines of this job" panel showed only the QPM's half of a job's story. The client's two lines (submitted job ..., job ... completed after ...) never left the client process.

Why

A record is dropped at its logger when it is below the logger's effective level, before any handler sees it, and a logger with no level of its own takes the root's. A DEFw service opens its root logger, a Qiskit client out of the box has it at Python's default WARNING, and the story lines are debug.

How

• The logs tier opens QFw's own qfw namespace to its level when the root logger would not, re-asserts it from logs_enabled() (a later DEFw level change can raise the root again), and restores the namespace's level on shutdown.
• Nothing else changes: every other logger still answers to the root logger's level, so turning the tier on does not start exporting a library's debug output from a client.
• Two unit tests: a quiet root (the client line is exported, other loggers stay local, the root is untouched, the namespace is restored) and an open root (the namespace is left alone). The mock fixture no longer opens the root logger, so tests/mock/test_instrumentation.py exercises the path a real client takes.
• One sentence in docs/design/benchmarking.md.

Checks

ci-syntax clean; tests/test_qfw_telemetry.py 19 passed; tests/mock/test_instrumentation.py 8 passed. A live check on the cluster (client lines under the trace in Grafana) follows in a comment.

[DougSO]

Live check on the reference cluster: the telemetry install rebuilt from this branch, 30 site-mode fake-IQM jobs (qfw_job_stream.sh --service-mode site --backend fake-iqm --jobs 30) with QFW_TELEMETRY_LOGS=debug, the client a plain srun Python process with nothing configuring its logging.

In Loki, this run
qfw-client lines	60, all stitched to a trace (2 per job: submitted, completed)
qfw-qpm lines	332, 238 stitched
Traces with both halves	30 of 30
Client lines other than the story	0 (the root logger's WARNING still gates everything else)
DEFw internals at debug	0

One trace (afac6eba93b241b0859cbd8d8f59cea2) on the QFw Trace page now reads: client submitted job ..., QPM received circuit ..., executing circuit ... on fake-iqm-20q via simulator, circuit ... completed ... after 1.2 ms, the QPM's run-path debug lines, then client job ... completed after 41.8 ms.