Quality: cold-start performance budget

Define a cold-start performance target for `func` and enforce it in
CI so regressions are caught early.

## Definition of Done

- [ ] Target cold-start time agreed (per platform)
- [ ] Benchmark harness lands
- [ ] CI fails on regression beyond budget
- [ ] Baseline numbers recorded

Start by reviewing how `func` is launched and how the repository's CI jobs are configured, then identify the platforms that need cold-start measurements. Done means the per-platform target is agreed, a benchmark harness and baseline numbers are recorded, and CI fails when a measurement exceeds the budget.

Written by the indexing model from the issue text.