feat(firecracker): port freeze/migrate off host Docker (PRD 0069 / #397)

The last host-Docker dependency in the Firecracker launch path. Freeze
and resume no longer touch the docker daemon, so the backend needs
firecracker + KVM only — completing #348.

Freeze: stream the guest rootfs over SSH straight into a persistent
committed-rootfs.tar (the resumable/migratable artifact) instead of
round-tripping through `docker build` from a scratch image. Written to
a .partial sibling and atomically renamed so a failed freeze leaves no
truncated artifact.

Resume: extract the snapshot tar into a cached base dir and feed it to
the existing rootless `mke2fs -d` pipeline, replacing the
`docker create` + `docker export | tar` path. Recreate the
proc/sys/dev/run mount points the freezer excludes so the guest init
can mount them.

`util.build_base_rootfs_dir` / `docker_image_id` stay — they still back
the opt-in BOT_BOTTLE_INFRA_BUILD=local dev path and off-host
publish_infra, which are out of scope.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

bot_bottle/bottle_state.py

@@ -44,6 +44,7 @@ from .paths import bot_bottle_root
 _STATE_SUBDIR = "state"
 _PER_BOTTLE_DOCKERFILE_NAME = "Dockerfile"
 _COMMITTED_IMAGE_NAME = "committed-image"
+_COMMITTED_ROOTFS_NAME = "committed-rootfs.tar"
 _TRANSCRIPT_SUBDIR = "transcript"
 # Per-daemon scratch subdirs. PRD 0018 chunk 2: bind-mount sources
 # live here so chunk 3's `docker compose up` can find them at stable
@@ -200,6 +201,15 @@ def committed_image_path(identity: str) -> Path:
     return bottle_state_dir(identity) / _COMMITTED_IMAGE_NAME
 
 
+def committed_rootfs_path(identity: str) -> Path:
+    """Where the Firecracker freezer stores a snapshot of the bottle's
+    guest rootfs (a plain tar). This is the resumable/migratable artifact
+    the Firecracker backend boots from — no Docker image involved. The
+    matching `committed-image` state file records that a snapshot exists
+    (and its path); `resume` boots from this tar when both are present."""
+    return bottle_state_dir(identity) / _COMMITTED_ROOTFS_NAME
+
+
 def write_committed_image(identity: str, image_tag: str) -> Path:
     """Persist the committed image tag for `identity`. The next
     `cli.py resume <identity>` will boot from this image instead of
@@ -354,6 +364,7 @@ __all__ = [
     "cleanup_state",
     "clear_preserve_marker",
     "committed_image_path",
+    "committed_rootfs_path",
     "egress_state_dir",
     "git_gate_state_dir",
     "is_preserved",