[Housekeeping] Separate compilation costs from runtime and streamline CI

[lrnv]

Scope

Measure compilation cost separately from warm numerical execution and use that evidence to streamline CI. Avoid ineffective sample-size reductions when compilation dominates.

Assess per-file timing, representative first-call versus warm-call costs, test sharding, Julia-version duplication, and the number of concrete type combinations compiled by generic tests.

Acceptance criteria

• baseline per-file and total CI timings;
• identification of compilation-bound versus runtime-bound files;
• proposed changes tied to measured bottlenecks;
• equivalent functional coverage across the CI matrix;
• before/after wall-time comparison on GitHub Actions.