feat(smolmachines): backend skeleton + Smolfile/gvproxy renderers (PRD 0023 chunk 1)

Ships the smolmachines backend's prepare side: subpackage layout,
`_BACKENDS` registration under "smolmachines", preflight check
for `smolvm` + `gvproxy` on PATH, and the two config-file
renderers (Smolfile TOML + gvproxy YAML). Launch raises
NotImplementedError until chunk 2.

New module layout (mirrors backend/docker/):
  claude_bottle/backend/smolmachines/
    __init__.py            re-exports SmolmachinesBottleBackend
    backend.py             SmolmachinesBottleBackend façade
    bottle.py              SmolmachinesBottle stub (NotImpl until ch2)
    bottle_plan.py         SmolmachinesBottlePlan + .print()
    bottle_cleanup_plan.py SmolmachinesBottleCleanupPlan stub
    prepare.py             resolve_plan: writes both config files
    smolfile.py            TOML renderer (stdlib, no tomli_w dep)
    gvproxy_config.py      YAML renderer (same shape as pipelock_yaml)
    util.py                preflight + per-slug subnet + loopback port

The renderers are pure functions. `resolve_plan` runs the
preflight, allocates one host-side loopback port per active
sidecar (pipelock always; git-gate / supervise conditional),
derives a per-slug gvproxy subnet (hash-mod-254, skipping the
docker-default 17), and writes:

  - <stage>/gvproxy.yaml: subnet + DNS rule resolving only
    `proxy.internal` + port_forwards (one per active sidecar).
  - <stage>/smolfile.toml: guest command/env + virtio-net device
    backed by gvproxy's unixgram socket. No TSI flags — see
    PRD 0023 "Why gvproxy, not TSI".

The agent's HTTPS_PROXY etc. point at `proxy.internal:<gateway-
port>` so the guest dials through gvproxy. gvproxy resolves only
`proxy.internal` → the gateway IP, and forwards exactly the
listed ports to the host-side sidecar bundle (PRD 0024); every
other destination — host LAN, host loopback, public internet
directly — is unreachable by construction.

29 new unit tests covering renderer correctness, subnet
derivation stability + collision-avoidance, loopback port
allocation, and preflight error paths. Full unit suite: 532
passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/smolmachines/bottle_plan.py

@@ -0,0 +1,64 @@
+"""SmolmachinesBottlePlan — concrete BottlePlan for the smolmachines
+backend (PRD 0023).
+
+Chunk 1 fields: slug, smolfile_path, gvproxy_config_path, gvproxy
+subnet + socket, and the per-bottle port map. VM lifecycle fields
+(machine name, OCI archive path, etc.) land in later chunks as the
+launch flow grows."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from pathlib import Path
+
+from ...log import info
+from .. import BottlePlan
+
+
+@dataclass(frozen=True)
+class SmolmachinesBottlePlan(BottlePlan):
+    """Resolved fields the launch step needs to bring up the bottle.
+
+    Inherits `spec` and `stage_dir` from BottlePlan."""
+
+    slug: str
+    smolfile_path: Path
+    gvproxy_config_path: Path
+    gvproxy_socket: Path
+    gvproxy_subnet: str
+    gvproxy_gateway: str
+    # Daemon name → host-side loopback port the bundle binds.
+    # Always includes "pipelock"; "git-gate" and "supervise"
+    # conditional on the bottle's manifest.
+    host_port_map: dict[str, int]
+
+    def print(self, *, remote_control: bool) -> None:
+        """Compact y/N preflight for the smolmachines path. Mirrors
+        the docker preflight's layout so operators don't have to
+        learn two formats."""
+        del remote_control  # not surfaced in the compact summary
+        spec = self.spec
+        manifest = spec.manifest
+        agent = manifest.agents[spec.agent_name]
+        bottle = manifest.bottle_for(spec.agent_name)
+
+        info(f"backend: smolmachines")
+        info(f"agent:   {spec.agent_name}")
+        info(f"bottle:  {agent.bottle}")
+        info(f"slug:    {self.slug}")
+        info(f"gvproxy: {self.gvproxy_gateway} on {self.gvproxy_subnet}")
+
+        env_names = sorted(bottle.env.keys())
+        skills = list(agent.skills)
+        upstreams = [g.Name for g in bottle.git]
+        routes = [r.host for r in bottle.egress.routes]
+        info(f"env:     {', '.join(env_names) if env_names else '(none)'}")
+        info(f"skills:  {', '.join(skills) if skills else '(none)'}")
+        info(f"git:     {', '.join(upstreams) if upstreams else '(none)'}")
+        info(f"routes:  {', '.join(routes) if routes else '(none)'}")
+        info(f"smolfile:       {self.smolfile_path}")
+        info(f"gvproxy config: {self.gvproxy_config_path}")
+        info(
+            "(chunk 1 of PRD 0023: prepare-only — launch is "
+            "not yet implemented)"
+        )