Benchmark julia-actions/cache for CI runtime

[bernalde]

Summary

• Adds julia-actions/cache@v3 after julia-actions/setup-julia@v3 and before julia-actions/julia-buildpkg@v1.
• Leaves package code and tests unchanged.
• Warm-cache benchmark meets the issue 23 material improvement threshold.

Validation

• python -c "import pathlib, yaml; yaml.safe_load(pathlib.Path('.github/workflows/ci.yml').read_text())": passed
• CI run 27127936887 attempt 1: passed as the cold-cache run.
• CI run 27127936887 attempt 2: passed as the warm-cache run.

Benchmark
Baseline from issue 23: run 27072931870.

Job	Baseline	Cold attempt 1	Warm attempt 2	Warm vs baseline
Julia 1.10 Ubuntu	2m38s	2m56s	1m52s	-46s (-29%)
Julia 1 Ubuntu	3m47s	3m42s	1m52s	-115s (-51%)
Julia 1 Windows	4m17s	4m36s	2m02s	-135s (-53%)

Critical path: 4m17s baseline to 2m02s warm-cache, -135s (-53%).

Step Timings

Attempt	Job	Total	setup-julia	cache restore	julia-buildpkg	native C ABI	julia-runtest	cache save
Cold	Julia 1.10 Ubuntu	2m56s	5s	<1s	17s	31s	1m48s	10s
Cold	Julia 1 Ubuntu	3m42s	9s	<1s	12s	22s	2m51s	4s
Cold	Julia 1 Windows	4m36s	16s	1s	28s	skipped	3m24s	17s
Warm	Julia 1.10 Ubuntu	1m52s	5s	6s	10s	26s	51s	7s
Warm	Julia 1 Ubuntu	1m52s	8s	5s	9s	26s	53s	5s
Warm	Julia 1 Windows	2m02s	16s	14s	10s	skipped	1m02s	10s

Cache Evidence

• Cold attempt: all three jobs logged No cache found and then Cache saved successfully.
• Warm attempt: all three jobs logged Cache restored successfully from attempt 1 keys, then saved attempt 2 caches.

Closes #23

[bernalde]

Blocking issues: none.

Nonblocking issues: none.

Questions: none.

Tests run and outcomes:

• Verified fetched PR head 92a0d6939370e914de4d868e1d41be471c17e84b against the local checkout.
• Verified local merge-base diff matches GitHub-reported totals: 1 changed file, 1 insertion, 0 deletions.
• python -c "import pathlib, yaml; yaml.safe_load(pathlib.Path('.github/workflows/ci.yml').read_text())": passed.
• gh pr checks 24: passed for all three matrix jobs.
• Verified CI run 27127936887 attempt 1 passed as the cold-cache run and attempt 2 passed as the warm-cache run.
• Attempted julia --project=. -e 'import Pkg; Pkg.test()' locally; it did not reach tests because this checkout's Julia 1.10 environment reports the manifest was resolved with Julia 1.12 and cannot locate OpenSSL_jll. I am not treating that as a PR regression because this PR only changes CI configuration and the GitHub Julia 1.10/1/Windows matrix passed on the PR head.

Issue coverage:

• The change adds only julia-actions/cache@v3 after setup-julia and before julia-buildpkg, matching issue 23.
• Cold and warm runs confirm cache miss/save, then restore/save, across all matrix jobs.
• Warm critical path improves from the issue baseline 4m17s to 2m02s, satisfying the requested material-improvement threshold.

Merge-readiness: merge-ready from my review. This review is submitted as COMMENT because the current GitHub account authored the PR; formal approval must come from another maintainer.