feat(firecracker): add Linux microVM backend to replace smolmachines

Adds a Firecracker-based backend for Linux, providing mature KVM-based
microVM isolation to replace smolmachines/libkrun (issue #342, closes
the dead-end tracked in #332).

Architecture:
- Guest control over SSH (dropbear injected into the rootfs) on a
  point-to-point TAP link. `ssh -t` forwards SIGWINCH natively, so no
  resize bridge is needed.
- Networking: a one-time, root-provisioned pool of user-owned TAP
  devices (no shared bridge → no docker0/virbr0/cni0 collisions) plus a
  dedicated `table inet bot_bottle_fc` nftables table (independent of
  Docker/ufw/firewalld rules). `./cli.py firecracker setup` prints the
  host-appropriate config (NixOS module or sudo script).
- Rootfs: `docker export` → ext4 via `mke2fs -d` (rootless, no mount),
  cached by image digest; per-bottle SSH pubkey + IP passed via the
  kernel cmdline.
- Sidecar: reuses the Docker bundle, published on the slot's host TAP IP.
- Fail-closed isolation: TAP pool verified at preflight; the egress
  boundary is proven empirically post-boot (before the agent runs) by a
  canary probe — the VM must fail to reach the host directly, or launch
  is refused.

Linux hosts with Firecracker + KVM now default to this backend;
macOS stays on macos-container.

Not yet validated end-to-end on live hardware (requires the one-time
network pool). Unit tests + pyright pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G8p32HJgPoS1hLPWubbftM

bot_bottle/backend/__init__.py

@@ -26,8 +26,9 @@ backend exposes five methods:
 
 Selection is driven by `--backend` on `start` or BOT_BOTTLE_BACKEND
 (env var). When neither is set, compatible macOS hosts default to
-`macos-container`; other hosts default to `smolmachines`. Per PRD 0003
-the manifest does not carry a backend field; the host picks.
+`macos-container`; Linux hosts with Firecracker + KVM default to
+`firecracker`; otherwise `smolmachines`. Per PRD 0003 the manifest
+does not carry a backend field; the host picks.
 """
 
 from __future__ import annotations
@@ -542,6 +543,7 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
 # each backend module can pull BottleSpec / BottlePlan / BottleBackend
 # via `from . import ...` without hitting a partially-initialized module.
 from .docker import DockerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
+from .firecracker import FirecrackerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 from .macos_container import MacosContainerBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 from .smolmachines import SmolmachinesBottleBackend  # noqa: E402  # pylint: disable=wrong-import-position
 
@@ -557,6 +559,7 @@ from .freeze import CommitCancelled, Freezer, get_freezer  # noqa: E402  # pylin
 # unparameterized methods (prepare → plan → launch(plan), cleanup, etc.).
 _BACKENDS: dict[str, BottleBackend[Any, Any]] = {
     "docker": DockerBottleBackend(),
+    "firecracker": FirecrackerBottleBackend(),
     "macos-container": MacosContainerBottleBackend(),
     "smolmachines": SmolmachinesBottleBackend(),
 }
@@ -585,6 +588,8 @@ def get_bottle_backend(
 def _default_backend_name() -> str:
     if has_backend("macos-container"):
         return "macos-container"
+    if has_backend("firecracker"):
+        return "firecracker"
     return "smolmachines"