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/__init__.py

@@ -281,6 +281,7 @@ 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
+from .smolmachines import SmolmachinesBottleBackend  # noqa: E402
 
 
 # The dict is heterogeneous: each value is a BottleBackend specialized
@@ -289,6 +290,7 @@ from .docker import DockerBottleBackend  # noqa: E402
 # unparameterized methods (prepare → plan → launch(plan), cleanup, etc.).
 _BACKENDS: dict[str, BottleBackend[Any, Any]] = {
     "docker": DockerBottleBackend(),
+    "smolmachines": SmolmachinesBottleBackend(),
 }