refactor: separate engine, collector, mutator, and fleet into independent subpackages

- decnet/engine/ — container lifecycle (deploy, teardown, status); _kill_api removed
- decnet/collector/ — Docker log streaming (moved from web/collector.py)
- decnet/mutator/ — mutation engine (no longer imports from cli or duplicates deployer code)
- decnet/fleet.py — shared decky-building logic extracted from cli.py

Cross-contamination eliminated:
- web router no longer imports from decnet.cli
- mutator no longer imports from decnet.cli
- cli no longer imports from decnet.web
- _kill_api() moved to cli (process management, not engine concern)
- _compose_with_retry duplicate removed from mutator

decnet/collector/__init__.py

@@ -0,0 +1,13 @@
+from decnet.collector.worker import (
+    is_service_container,
+    is_service_event,
+    log_collector_worker,
+    parse_rfc5424,
+)
+
+__all__ = [
+    "is_service_container",
+    "is_service_event",
+    "log_collector_worker",
+    "parse_rfc5424",
+]