refactor(backend): thin the docker/firecracker/macos-container package inits

The three concrete-backend package inits eagerly imported their
`*BottleBackend` (which pulls the whole framework), so importing any leaf under
them (e.g. `backend.docker.util`) paid ~78 modules. Give each the same thin
`__getattr__` treatment as the top-level backend package: re-export the public
class(es) lazily on first access, with a TYPE_CHECKING block for checkers.

`import bot_bottle.backend.docker.util` drops from 78 -> 6 bot_bottle modules;
the docker/firecracker/macos-container packages themselves are ~3 each. All
`from bot_bottle.backend.docker import DockerBottleBackend` call-sites and the
lazy selection path keep working.

This is what makes backend leaves cheap — and unblocks moving docker_cmd into
backend/docker/util if we want.

Full unit suite green (2243).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/firecracker/__init__.py

@@ -1,7 +1,25 @@
-"""Firecracker backend: Linux KVM microVM isolation (issue #342)."""
+"""Firecracker backend: Linux KVM microVM isolation (issue #342).
+
+Thin by design: `FirecrackerBottleBackend` is re-exported lazily via
+`__getattr__`, so importing a leaf module under this package doesn't drag the
+backend (and the framework it pulls) into memory.
+"""
 
 from __future__ import annotations
 
-from .backend import FirecrackerBottleBackend
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from .backend import FirecrackerBottleBackend
+
+
+def __getattr__(name: str) -> Any:
+    if name == "FirecrackerBottleBackend":
+        from .backend import FirecrackerBottleBackend
+
+        globals()[name] = FirecrackerBottleBackend
+        return FirecrackerBottleBackend
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
 
 __all__ = ["FirecrackerBottleBackend"]