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/util.py

@@ -0,0 +1,84 @@
+"""Slug / preflight helpers for the smolmachines backend (PRD 0023).
+
+Backend-specific utilities the prepare/launch flow reaches for.
+Kept in its own module so the renderers can be unit-tested without
+importing the docker subprocess paths."""
+
+from __future__ import annotations
+
+import hashlib
+import os
+import shutil
+import socket
+from contextlib import closing
+
+from ...log import die
+
+
+def smolmachines_preflight() -> None:
+    """Ensure the binaries the smolmachines backend shells out to are
+    on PATH. Called from the backend's `_resolve_plan` before any
+    file writes; gives the operator a clear pointer rather than a
+    cryptic FileNotFoundError later.
+
+    `smolvm` drives the libkrun microVM lifecycle. `gvproxy`
+    (gvisor-tap-vsock) is the userspace TCP/IP stack the guest's
+    virtio-net device hooks into."""
+    missing: list[tuple[str, str]] = []
+    if shutil.which("smolvm") is None:
+        missing.append((
+            "smolvm",
+            "brew install smolmachines-dev/smolmachines/smolmachines  "
+            "# or build from https://smolmachines.com/",
+        ))
+    if shutil.which("gvproxy") is None:
+        missing.append((
+            "gvproxy",
+            "go install "
+            "github.com/containers/gvisor-tap-vsock/cmd/gvproxy@latest  "
+            "# requires Go + ensure $GOPATH/bin on PATH",
+        ))
+    if not missing:
+        return
+    lines = [
+        f"CLAUDE_BOTTLE_BACKEND=smolmachines requires the following "
+        f"binar{'y' if len(missing) == 1 else 'ies'} on PATH:",
+    ]
+    for name, install in missing:
+        lines.append(f"  - {name}    install: {install}")
+    die("\n".join(lines))
+
+
+def smolmachines_gvproxy_subnet(slug: str) -> tuple[str, str]:
+    """Derive a per-bottle subnet + gateway IP from the slug.
+
+    Returns `(subnet_cidr, gateway_ip)`. The third octet comes from
+    SHA-256 of the slug mod 254 (1..254, skipping 0 and 255 + the
+    127.x.x.x range) — collision-free for any reasonable concurrent-
+    bottle count and stable across `start` / `resume` of the same
+    bottle. Bottles that DO collide would fail at gvproxy bringup;
+    `launch.py` (chunk 2) detects the conflict and surfaces a clear
+    error rather than silently reusing another bottle's gateway."""
+    digest = hashlib.sha256(slug.encode("utf-8")).digest()
+    octet = (digest[0] % 254) + 1
+    # Skip docker-default 17 to dodge the usual second-bridge
+    # collision. Operators with docker's default bridge at
+    # 172.17.x.x — common Linux setup — would otherwise see a
+    # collision the moment they run a parallel docker bottle.
+    if octet == 17:
+        octet = 18
+    subnet = f"192.168.{octet}.0/24"
+    gateway = f"192.168.{octet}.1"
+    return subnet, gateway
+
+
+def allocate_loopback_port() -> int:
+    """Bind / release dance to grab a free TCP port on 127.0.0.1.
+
+    Race window: a parallel allocator could grab the same port
+    between our close and the bundle's bind. The window is small
+    enough that chunk 1 doesn't address it; chunk 3 will add a
+    retry loop if it shows up in practice."""
+    with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
+        s.bind(("127.0.0.1", 0))
+        return int(s.getsockname()[1])