feat(observability): implement Event System Domain Event Taxonomy (full EventType enum + DomainEvent model)

Verified and completed the Event System Domain Event Taxonomy implementation.

Gap analysis:
- EventType enum (38 types across 12 domains): ALREADY EXISTED, verified complete
- DomainEvent Pydantic model (all 9 fields): ALREADY EXISTED, verified complete
- EventBus Protocol (emit + subscribe): ALREADY EXISTED, verified complete
- ReactiveEventBus (RxPY Subject, stream property): ALREADY EXISTED, verified
- LoggingEventBus (structlog-based): ALREADY EXISTED, verified complete

Added:
- In-memory audit_log on ReactiveEventBus (list[DomainEvent] with defensive copy)
- Emit ordering aligned with specification: RxPY stream push, then audit append,
  then handler dispatch (§Event System)
- Clarified audit_log docstring: volatile in-memory log, not durable SQLite
  persistence (durable persistence wired separately via audit service layer)
- Behave feature: features/observability/event_system_taxonomy.feature (36 scenarios)
- Step definitions: features/steps/event_system_taxonomy_steps.py
- Robot integration test: robot/event_system_taxonomy_integration.robot (13 test cases)
- Robot helper: robot/helper_event_system_taxonomy.py (13 subcommands)
- ASV benchmark: benchmarks/bench_event_bus.py (5 benchmark suites)
- vulture_whitelist.py: added audit_log property

Code review fixes applied:
- Reordered emit() to match spec: on_next → audit_log → handlers (was: audit → on_next → handlers)
- Clarified audit_log docstring to distinguish volatile in-memory log from spec SQLite table
- Fixed benchmark TaxonomyAuditLogSuite: parameterized setup instead of inline construction
- Added teardown to TaxonomyLoggingEmitSuite to restore logging.disable(NOTSET)
- Clear _received list between benchmark iterations to reduce noise
- Catch specific pydantic.ValidationError in frozen model mutation tests
- Consolidated duplicate singular/plural step definitions
- Tightened EventType count threshold from >=30 to >=38

Quality gates:
- lint: PASSED
- typecheck: PASSED (0 errors)
- unit_tests: 36/36 new scenarios PASSED (pre-existing 12 flaky failures unchanged)
- integration_tests: 1483/1483 PASSED
- security_scan: PASSED
- dead_code: PASSED

ISSUES CLOSED: #587

docs/reference/event_bus.md

@@ -93,7 +93,7 @@ satisfies it:
 ```python
 from cleveragents.infrastructure.events import EventBus, ReactiveEventBus
 
-bus = ReactiveEventBus()
+bus = ReactiveEventBus(max_audit_log_size=5000)  # optional retention cap
 assert isinstance(bus, EventBus)  # True
 ```
 
@@ -124,7 +124,7 @@ bus = ReactiveEventBus()
 # Subscribe a plain callback
 bus.subscribe(EventType.PLAN_CREATED, lambda e: print("Plan created:", e.plan_id))
 
-# Use the raw RxPY stream for advanced operators
+# Use the read-only RxPY stream for advanced operators
 bus.stream.pipe(
     rx.operators.filter(lambda e: e.plan_id is not None),
 ).subscribe(lambda e: print("Plan event:", e))
@@ -136,7 +136,8 @@ bus.emit(DomainEvent(event_type=EventType.PLAN_CREATED, plan_id="01ABC..."))
 - `emit(event)` pushes to the RxPY Subject **and** calls all type-specific
   handlers synchronously.
 - `subscribe(event_type, handler)` registers a callback for one event type.
-- `stream` exposes the raw `rx.Observable` for RxPY operators.
+- `stream` exposes a read-only `rx.Observable` view for RxPY operators.
+- `clear_audit_log()` clears retained in-memory events when needed.
 
 ---