docs: consolidate docs/testing — 17 files to STRATEGY.md plus pointers #2339

Rail R5 of the journey-coverage deck.

Summary

Reduce docs/testing/ to STRATEGY.md plus pointers; archive the rest to docs/archive/testing/.

Context

Testing guidance is currently scattered across five places — docs/testing/ (17 files, several stale, including TESTING_FRAMEWORK_COMPLETE.md and UI_TESTING_GAP_ANALYSIS.md), tests/README.md, src/frontend/e2e/README.md, tests/registry.json, and two epics. Scattered guidance is why the same gap gets rediscovered every few months.

Three files, or it fragments again within a quarter.

Acceptance Criteria

• docs/testing/STRATEGY.md exists and carries the method, the tiers, and the acceptance bar for a harness
• The invariant catalog stays where it is and is linked, not absorbed
• Stale and dated files move to docs/archive/testing/ — nothing is deleted outright
• Anything still needed from tests/README.md / src/frontend/e2e/README.md is linked from STRATEGY.md rather than copied
• No document in docs/testing/ contradicts another after the pass

Technical Notes

Do this last of the rails — STRATEGY.md should describe the tier and catalog as built, not as planned.

Journey-coverage process, filed 2026-08-20 from the operator session of 08-19/20. Epic #1850 (rechartered). R1–R5 ship before any journey issue moves to status-in-progress. Journey records reference invariant IDs in docs/testing/orchestration-invariant-catalog.md — never restate them.