Summary

• add a new root-level TESTING.md that defines NullHub's incremental testing strategy and aligns contributor expectations with the NullClaw-style testing model
• document the current subsystem coverage map so future testing work can target the weakest areas first instead of adding tests blindly
• lay out an isolated PR roadmap for backend gate stabilization, shared fixtures, high-risk backend coverage, integration tests, UI tests, browser E2E, and CI enforcement
• link the README development section to TESTING.md so contributors can find the testing strategy from the main landing page

Validation

• git diff --check
• manually verified README.md references TESTING.md correctly
• manually verified the coverage map and roadmap against the current src/, ui/, and tests/ layout on main

Notes

• this PR is docs-only and intentionally does not change CI, hooks, or runtime behavior
• the coverage map reflects the current main branch rather than in-flight feature branches so it stays clean for upstream review
• follow-up test PRs should stay small and use this document as the sequencing guide