Fork census

Read the fork-budget reference before changing process costs. Its historical timings are evidence of earlier measurements, not current baselines. All repository paths below are relative to the repository root.

1. Identify the actual path: cold start, sequential run, parallel worker, lifecycle hook, report, or coverage. Read the matching existing fixture in tests/acceptance/bashunit_coldstart_forks_test.sh, tests/acceptance/bashunit_run_forks_test.sh, or tests/acceptance/bashunit_coverage_forks_test.sh. Include the options and hooks the change exercises.
2. For a timing claim, measure current main before editing and record its commit, Bash version, OS, fixture, options, and sampling method. Land any new benchmark harness before the PR that measures a delta. Alternate baseline and candidate rounds in the same location and repeat anomalous readings.
3. Use the reference's PATH-shim census for binary invocations. Resolve real binaries before adding shims; startup-pinned binaries bypass them. Account separately for shell-function subshells. Use traces to locate sites, not raw trace-line totals to count forks. Skip unreliable PATH-shim checks on Git Bash.
4. Add a failing budget regression using the existing fixture patterns, then make the smallest change. Measure parallel costs with a parallel fixture and compare supported Bash versions. Never replace a fork with pure Bash based only on a microbenchmark.
5. Run the matching acceptance file with ./bashunit --no-parallel tests/acceptance/<existing-file>. These tests select their own internal sequential or parallel census. Preserve the checked budgets; report counts, timing samples, and limits. The main session runs the full quality gate after the complete change set.