test: add comprehensive test suite (180+ tests, 20 benchmarks)

[namuan]

Summary

Adds a rigorous automated testing suite for the durable workflow engine, expanding coverage from 25 tests to 180+ tests + 20 benchmarks across 9 new test files organized by domain.

New test files

File	Tests	Coverage
engine_schema_test.go	16	Table existence, columns, indexes, defaults, PRAGMAs, foreign keys, STRICT
engine_state_machine_test.go	30	All state transitions, invariants, version tracking, lease lifecycle, ID edge values
engine_concurrency_test.go	15	Goroutine racing leases, worker pools, concurrent signals/timers/steps, fairness
engine_edge_cases_test.go	30	Nil payloads, large step numbers, nonexistent workflows, empty queues, timer edges
engine_recovery_test.go	10	Crash with lease, mid-step, multi-workflow, signal/timer persistence, reopen
engine_event_sourcing_test.go	15	Sequence continuity, all event types, monotonicity, isolation, state replay
engine_complex_scenarios_test.go	12	Full saga with retry, signal triggers, chained timers, interleaved workflows
engine_scheduler_test.go	14	Timer fire, future protection, lease expiry, continuous run, cancellation
engine_benchmarks_test.go	20	Throughput for create/acquire/step/signal/timer, concurrent variants, full lifecycle

Other changes

• Added QueryCompleted() method to Engine (follows existing pattern)
• Updated README with testing documentation section
• All tests pass with -race enabled