refactor(smolmachines): decompose launch(), add wait_exec_ready, file-lock allocate() (PRD 0032)

Decompose the 207-line launch() into six named helpers: _allocate_resources,
_mint_certs, _start_bundle, _discover_urls, _launch_vm, _init_vm. Each has
explicit inputs/outputs and is independently testable.

Replace time.sleep(1.5) with smolvm.wait_exec_ready(), which polls
`machine exec true` with exponential backoff. Exits as soon as the exec
channel is ready; dies loudly with a timeout message instead of silently
leaving the VM in an unknown state.

File-lock loopback_alias.allocate() with fcntl.flock(LOCK_EX) so concurrent
bottle launches can't race on docker state and claim the same alias.

bot_bottle/backend/smolmachines/smolvm.py

@@ -27,10 +27,13 @@ from __future__ import annotations
 
 import shutil
 import subprocess
+import time
 from dataclasses import dataclass
 from pathlib import Path
 from typing import Mapping, Sequence
 
+from ...log import die
+
 
 _SMOLVM = "smolvm"
 
@@ -197,6 +200,30 @@ def machine_exec(
     )
 
 
+def wait_exec_ready(name: str, *, timeout: float = 5.0) -> None:
+    """Poll `machine exec true` until exit 0 or `timeout` elapses.
+
+    Replaces `time.sleep(1.5)` after `machine_start`: libkrun's exec
+    channel needs a brief warm-up before back-to-back exec calls are
+    safe. Polling exits as soon as the channel is ready and fails
+    loudly if the VM never responds."""
+    deadline = time.monotonic() + timeout
+    delay = 0.1
+    while time.monotonic() < deadline:
+        r = machine_exec(name, ["true"])
+        if r.returncode == 0:
+            return
+        remaining = deadline - time.monotonic()
+        if remaining <= 0:
+            break
+        time.sleep(min(delay, remaining))
+        delay = min(delay * 2, 0.5)
+    die(
+        f"smolvm machine {name!r}: exec channel not ready after "
+        f"{timeout:.0f}s — VM may have failed to boot."
+    )
+
+
 def machine_cp(src: str, dst: str) -> None:
     """`smolvm machine cp SRC DST`. Path syntax: `machine:path` to
     reference a path inside the VM, bare path for the host. Both