The fixed four jobs were safe on 8 GB but too many for 4 GB, and slower than
needed on large machines. `cargo xtask ci` now uses one build job per 2 GB of
installed RAM, capped at the CPU count, and 4 if the RAM can't be read
(/proc/meminfo, Win32_ComputerSystem, hw.memsize). With line-table debuginfo,
4 jobs measured about 3 GB of RAM in use and 2 jobs about 1.5 GB.
Test threads follow but stop at 4: more barely shortens CI, and at 16 threads
on a loaded machine the wall-clock frame-budget test
offline_originals_never_block_a_frame failed (one 303 ms frame). Explicit
CARGO_BUILD_JOBS / RUST_TEST_THREADS still win.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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