ci: cap section container threads at 2 (CARGO_BUILD_JOBS/RAYON/nextest) — intel load/cores 6.1 vs gate 1.2 (P1b) #5746

Operator order P1b (relayed by the cop): thread cap = cores / jobs per host. Measured on intel 10:35Z: 32 cores, 14 busy workers, 26 containers, load 195/153/131 (ratio 6.1; gate <=1.2). Runner .env has JOBS=4, but ci/sections.yml passes -e CARGO_BUILD_JOBS=8 into 10 section containers (11 lines); RAYON_NUM_THREADS unset. Fix: CARGO_BUILD_JOBS, RAYON_NUM_THREADS, NEXTEST_TEST_THREADS, RUST_TEST_THREADS = 2 in those docker runs. Report load/cores before and after. Note: /mnt/nvme-raid0/cop-inbox/handoff/p1b-thread-cap-intel-1035Z.md