📚 Add testing principles to serena doc

.serena/memories/backend/core.md

@@ -106,3 +106,4 @@ IMPORTANT: all CLI commands must be executed from the `backend/` subdirectory.
 * **Coverage:** If code is added or modified in `src/`, corresponding tests in `test/backend_tests/` must be added or updated.
 * **Isolated run:** `clojure -M:dev:test --focus backend-tests.my-ns-test` for a specific test namespace.
 * **Regression run:** `clojure -M:dev:test` to ensure no regressions in related functional areas.
+* **Principles:** Cross-cutting testing principles, anti-patterns, and verification checklist: `mem:common/testing-principles`.

.serena/memories/common/core.md

@@ -49,6 +49,7 @@ Components, variants, and debugging:
 Text and tests:
 - Shared text data conversion, DraftJS compatibility, modern text content, and derived position data: `mem:common/text-subtleties`.
 - Common test commands, helper conventions, production-path test mutations, and runtime coverage choices: `mem:common/testing`.
+- Cross-cutting testing principles, anti-patterns, and verification checklist: `mem:common/testing-principles`.
 
 ## Areas without focused memories
 

.serena/memories/common/testing-principles.md

@@ -0,0 +1,47 @@
+# Testing Principles (Cross-Cutting)
+
+Shared testing principles for all modules (backend, frontend, common, render-wasm, plugins). Module-specific commands and helpers are in each module's own testing memory.
+
+## Core Principles
+
+- **Test State, Not Interactions** — assert on outcomes, not method calls; survives refactoring
+- **DAMP over DRY** — tests are specifications; duplication OK if each test is self-contained and readable
+- **Prefer Real Implementations** — hierarchy: Real > Fake > Stub > Mock; mock only at boundaries (network, filesystem, email)
+- **Arrange-Act-Assert** — every test: setup / action / verify
+- **One Assertion Per Concept** — each test verifies one behavior; split compound assertions
+- **Descriptive Test Names** — names read like specifications
+
+## Anti-Patterns
+
+- Testing implementation details → breaks on refactor
+- Flaky tests (timing, order-dependent) → erode trust
+- Mocking everything → tests pass, production breaks
+- No test isolation → pass individually, fail together
+- Testing framework code → waste of time
+- Snapshot abuse → nobody reviews, break on any change
+
+## Verification Checklist
+
+After completing any implementation:
+- Every new behavior has a corresponding test
+- All tests pass for touched modules
+- Bug fixes include a reproduction test that failed before the fix
+- Test names describe the behavior being verified
+- No tests were skipped or disabled
+
+## Red Flags
+
+- Writing code without corresponding tests
+- Tests that pass on the first run (may not be testing what you think)
+- Bug fixes without reproduction tests
+- Tests that test framework behavior instead of app behavior
+- Skipping tests to make the suite pass
+
+## Rationalizations
+
+- "I'll write tests after" → you won't; they'll test implementation not behavior
+- "Too simple to test" → simple gets complicated; test documents expected behavior
+- "Tests slow me down" → they speed up every future change
+- "I tested manually" → manual testing doesn't persist
+- "Code is self-explanatory" → tests ARE the specification
+- "Just a prototype" → prototypes become production; test debt accumulates

.serena/memories/frontend/core.md

@@ -52,6 +52,7 @@ Diagnostics and validation:
 - Source-edit compile/hot-reload diagnostics: `mem:frontend/compile-diagnostics`.
 - Runtime crash recovery: `mem:frontend/handling-crashes`.
 - Tests and live verification: `mem:frontend/testing`.
+- Cross-cutting testing principles and anti-patterns: `mem:common/testing-principles`.
 - Real pointer/keyboard gesture reproduction: `mem:frontend/playwright-gestures`.
 
 ## Areas without focused memories