ci: start ClickHouse while the test archives download

[MarcusKainth]

Stacked on #588 (ci/sim-groups-start-early); the diff against that branch is this one commit.

What this changes, and why

A test job runs make up first and waits 15 to 27 s for the server to report healthy. Only then does it install nextest and download the archives, and neither of those needs the server.

Both test jobs now start docker compose up -d --wait in the background as their first step after checkout, and write its log and exit status under RUNNER_TEMP. Before the suites, "Wait for ClickHouse" waits for that status, prints the log and runs make up. make up returns at once when the server answers, and starts the server itself when it doesn't, so a failed background start still fails loudly in that step.

differential-smoke keeps its plain make up, because its next step is a release build that takes longer than the server does to start.

Evidence

$ make actionlint zizmor; echo "exit=$?"
No findings to report. Good job! (12 suppressed)
exit=0

Step times on this PR's CI run: pending.

Invariants

None. CI step order only.

Spec impact

• None. No contract in SPEC.md is touched

Checks

• make gates
• No AI attribution trailers in the commits

Written mostly by Claude Opus 5.5.

[MarcusKainth]

Closing: the measurement doesn't support it. Median seconds from a test job's start to its suites step: 22.5 on #588's run 36125404729 (plain make up first), 24.5 on this PR's run 36125470574 (ClickHouse started in the background). After the smaller archives, installing nextest and downloading take about 5 s, so the background start has nothing to overlap. "Wait for ClickHouse" took 6 to 24 s here, against 14 to 28 s for "Start the pinned ClickHouse" in #588. The branch ci/clickhouse-in-background stays if anyone wants it.