fix(attendance): preserve frozen attendance boundaries

docs/tests/unit/attendance-checkout-eligibility.md

@@ -0,0 +1,92 @@
+# Test Evidence: Checkout eligibility and stable conflict outcomes
+
+- **Test type:** Unit
+- **Requirement IDs:** `ATT-007`, `ATT-008`, `ATT-010`, `ATT-012`
+- **Scenario IDs:** `AC-ATT-003`, `AC-ATT-004`
+- **Test class/method:** `com.lab.labtimesheet.feature.attendance.service.AttendanceApplicationServiceTest#rejectsCheckoutWhenInternIsNoLongerEligibleForPersistedWorkDate`, `#translatesConcurrentCheckInUniqueConflictToStableDuplicateRejection`, `#translatesConcurrentCheckoutVersionConflictToStableDuplicateRejection`
+- **Implementation commit:** `pending`
+
+## Protected behavior
+
+Checkout revalidates active internship eligibility for the attendance row's
+persisted work date. A terminal Intern cannot checkout after checking in.
+Database uniqueness and optimistic-lock races are translated to stable duplicate
+punch rejection codes instead of leaking persistence exceptions.
+
+## Test method
+
+Plain JUnit and Mockito drive the production transactional application service
+with a fixed Clock, attached policy, persisted row, AccountService eligibility,
+and repository exceptions. Account state remains behind its public service API.
+
+## Hand-derived expected result
+
+False date-aware eligibility returns `INACTIVE_INTERN` before raw checkout is
+saved. A check-in uniqueness race returns `ALREADY_CHECKED_IN`; a checkout
+version race returns `ALREADY_CHECKED_OUT`.
+
+## RED
+
+**Command**
+
+```text
+export JAVA_HOME=/opt/homebrew/opt/openjdk@25
+export PATH="$JAVA_HOME/bin:$PATH"
+./mvnw -Dtest=AttendanceApplicationServiceTest#rejectsCheckoutWhenInternIsNoLongerEligibleForPersistedWorkDate test
+```
+
+**Observed result**
+
+```text
+Expected AttendanceException(INACTIVE_INTERN) but was NullPointerException after
+the service continued to save checkout without calling AccountService eligibility.
+Tests run: 1, Failures: 1, Errors: 0, Skipped: 0
+BUILD FAILURE
+Process exited 1.
+```
+
+The conflict regressions were also observed RED in the combined focused run:
+
+```text
+DataIntegrityViolationException: concurrent unique conflict
+ObjectOptimisticLockingFailureException: optimistic locking failed
+Both escaped AttendanceApplicationService instead of stable AttendanceException values.
+```
+
+## GREEN
+
+**Command**
+
+```text
+export JAVA_HOME=/opt/homebrew/opt/openjdk@25
+export PATH="$JAVA_HOME/bin:$PATH"
+./mvnw -Dtest=AttendanceApplicationServiceTest test
+```
+
+**Observed result**
+
+```text
+Tests run: 5, Failures: 0, Errors: 0, Skipped: 0
+BUILD SUCCESS
+Process exited 0.
+```
+
+## Affected suite
+
+**Command and result**
+
+```text
+export JAVA_HOME=/opt/homebrew/opt/openjdk@25
+export PATH="$JAVA_HOME/bin:$PATH"
+export DOCKER_HOST=unix:///Users/sechmachine/.orbstack/run/docker.sock
+./mvnw -Dtest='*Attendance*Test' test
+Tests run: 32, Failures: 0, Errors: 0, Skipped: 0
+BUILD SUCCESS
+Process exited 0.
+```
+
+## External-test boundaries
+
+The account platform has no Iteration 1 terminal-state mutation API, so the
+completed/withdrawn state is represented through its public date-aware eligibility
+result. The companion PostgreSQL concurrency test proves the real unique conflict.