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

tests/test_build.py

@@ -8,7 +8,13 @@ MODULES = [
     "decnet.cli",
     "decnet.config",
     "decnet.composer",
-    "decnet.deployer",
+    "decnet.engine",
+    "decnet.engine.deployer",
+    "decnet.collector",
+    "decnet.collector.worker",
+    "decnet.mutator",
+    "decnet.mutator.engine",
+    "decnet.fleet",
     "decnet.network",
     "decnet.archetypes",
     "decnet.distros",