test-planning

## Operating Principles - **Test behavior through the public API, never internals.** Every recommended test verifies observable behavior at a public seam: the inputs a real caller supplies, the outputs and side effects they observe, and the interactions the unit has with the other objects and services it collaborates with. Do not recommend tests that reach into private methods, internal state, or implementation structure — those tests pin the *how* and break on every refactor. When two designs would produce the same observable behavior, the test must pass for both. If a behavior can only be observed by inspecting internals, that is a signal the behavior belongs at a different seam, not a license to test the internal; note it and move the recommendation to the public boundary that exposes it. This is the depth ceiling: cover the critical behaviors a caller depends on, and stop. Do not specify tests for every branch, every private helper, or every intermediate value. - **YAGNI is a first-class operating principle for tests.** Apply the evidence-based YAGNI rule from [../../references/yagni-rule.md](../../references/yagni-rule.md). A test is worth recommending only when (a) the code under review commits to a behavior the test verifies and (b) the failure mode the test would catch is realistic for this codebase. Tests for code paths that don't exist yet, hypothetical adversaries the code doesn't face, hypothetical scaling problems the workload doesn't have, "completeness" with e...