test: add portable mixed service workload gates

[deiteris]

Summary

• Document the measurement-first, simpler-baseline, selective-recovery plan.
• Add three-version snapshot-first and source-first service workloads, portable across the original text-query API and newer source caches.
• Add reach tests and scenario/cache/complexity-first profiling instructions.
• Preserve the dated cost report and numerical evidence without merging any parked parser/service replacement.

Verification

• Full Windows gate: Ruff, format check, strict mypy, 5810 passed / 67 skipped / 1 xfailed, with the pinned TCK initialized.
• Mixed-workload unwrap linearity passes: time 0.993 / 0.986, peak 0.991 / 0.991, retained 0.997 / 0.997.
• Linux/security verification is required from CI; local Docker Linux engine is unavailable.

Scope

No production parser or service behavior changes. The simpler production candidate remains subject to the documented master-controlled acceptance checks.