fix(ci): Cap check-container cores and raise the timeout floor

[krlmlr]

Prepared with Claude Code (LLM-assisted).

Two remedies from the contention analysis of the revdepx validation run 32574134229, where all seven timeouts sat exactly at the 1200 s floor while the 4-core shards ran at 1-minute loads of 5–13.

_R_CHECK_LIMIT_CORES_=TRUE in every check container (check-half.sh, overridable through the environment). This is the value CRAN's own check machines use: a cluster or fork call asking for more than 2 workers errors instead of spawning them. Without it, a test suite that sizes itself from parallel::detectCores() sees the runner's 4 cores and fans out 4 workers per check — and with 4 concurrent checks per shard, that fan-out is what drove the load to 5–13 and produced the hot windows every timeout died in (the timed-out packages' windows ran at median load 7.5–11.4). Both halves of every comparison get the identical setting, so old-vs-new verdicts are unaffected; checks simply behave the way CRAN runs them.

Timeout floor 20 → 30 minutes (REVDEPX_TIMEOUT_MIN_MINUTES default). A saturated shard runs each check at roughly half speed — the measured inflation of queue-engine half-times over the pair engine's wall clock for the same 1,923 packages is 1.5× at the median, 1.8× at p90, and up to ~3× in the hottest windows. The old 20-minute floor killed four packages (orbital, PortfolioTesteR, SMMAL, xpect) that finish in 7–12 minutes uncontended. The cost is bounded: a genuinely hung check holds one worker ten extra minutes.

Both changes live in the shared revdepx core plus the revdep4.yaml defaults, so they apply to the merged queue engine now and to the pair engine (#2856) as-is on its merge — only revdep3.yaml's own || '20' floor default would want the same one-line bump on that branch.

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https://claude.ai/code/session_01UB7YutLzVWU7xCTYUvF3kF

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