thalarch-no-regressionlisted

# Thalarch No-Regression A working subsystem is evidence. Treat preservation of its important behavior as an explicit contract before optimizing or refactoring it. ## 1. Write the preservation contract first Before mutation, record a compact contract: ```text SURFACE: <subsystem> Must preserve: - <observable behavior/invariant> - <observable behavior/invariant> Suspected problem: - <specific hypothesis> Evidence: - PROVEN / SUPPORTED / INFERENCE / UNKNOWN / UNVERIFIED Action: - AUDIT ONLY / TARGETED MUTATION Verification: - <tests/runtime/profiling required> ``` Keep it task-local. Do not create bureaucracy or a permanent spec file unless the repository already uses one. ## 2. Unknown problem means audit first If the suspected defect/inefficiency is `UNKNOWN` or only weak inference: - inspect the actual implementation; - trace lifecycle/state/data flow; - measure or reproduce when feasible; - search for the concrete failure mechanism; - do not rewrite the subsystem merely because another project solved a similar issue differently. When the audit does not demonstrate the suspected problem, **preserve the implementation** and report the negative finding. A no-change outcome can be the correct engineering result. ## 3. Define blast radius List adjacent behavior that shares the changed state, data, or lifecycle. Examples: - cache change → cached/uncached reads, eviction, offline mode, downloads, prefetch, seek, recovery; - queue change → current item, duplicates