Add stdlib load-time regression CI check (macOS aarch64)

[IanButterworth]

Claude:

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Compares the Julia built by the pipeline (candidate) against the nightly associated with the merge-base commit (baseline). For each binary it re-precompiles every stdlib into an isolated, cache-free depot and measures load times, flagging clear regressions (relative and absolute thresholds).

• utilities/stdlib_load_time_regression.sh: driver that downloads/extracts both binaries, building the baseline from source if the merge-base nightly is missing, then runs the comparison. Only runs on macOS aarch64.
• compare_stdlib_load_times.jl: orchestrator. Per binary, generates a resolved stdlib Project/Manifest with the standard depot, precompiles the whole environment at once into a fresh depot, then measures load times in A/B/B/A/B/A order taking the per-stdlib minimum across repeats.
• measure_stdlib_load_times.jl: per-binary helper with generate/precompile/ measure modes; each measure runs one stdlib in its own process so previously-loaded deps don't skew timings.
• pipelines/main/misc/stdlib_load_time_regression.yml + launcher upload.