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.

tests/unit/test_smolmachines_loopback_alias.py

@@ -11,6 +11,7 @@ import json
 import sqlite3
 import subprocess
 import tempfile
+import threading
 import unittest
 from pathlib import Path
 from unittest.mock import patch
@@ -144,6 +145,55 @@ class TestAllocate(unittest.TestCase):
                 loopback_alias.allocate("demo-overflow")
 
 
+class TestAllocateLock(unittest.TestCase):
+    """allocate() on macOS acquires a file lock so concurrent calls
+    serialise rather than racing on docker state."""
+
+    def test_acquires_exclusive_lock_on_macos(self):
+        import fcntl as fcntl_mod
+        flock_calls: list[int] = []
+
+        def record_flock(fd, op):
+            flock_calls.append(op)
+
+        with tempfile.TemporaryDirectory() as tmp:
+            lock_path = Path(tmp) / "smolmachines.lock"
+            with patch.object(loopback_alias, "_is_macos", return_value=True), \
+                 patch.object(loopback_alias, "_ALLOC_LOCK_PATH", lock_path), \
+                 patch.object(loopback_alias, "_aliases_in_use", return_value=set()), \
+                 patch.object(loopback_alias.fcntl, "flock",
+                              side_effect=record_flock):
+                loopback_alias.allocate("demo")
+
+        self.assertIn(fcntl_mod.LOCK_EX, flock_calls)
+
+    def test_no_lock_on_linux(self):
+        # Linux early-returns before touching the lock file.
+        with patch.object(loopback_alias, "_is_macos", return_value=False), \
+             patch.object(loopback_alias.fcntl, "flock") as flock:
+            loopback_alias.allocate("demo")
+        flock.assert_not_called()
+
+    def test_sequential_allocations_with_shared_lock_are_serialised(self):
+        # Two sequential calls share the same lock file. The second
+        # call sees {127.0.0.16} in use (as if the first caller's
+        # docker run completed between the two lock acquisitions) and
+        # returns the next alias.
+        in_use_seq = [set(), {"127.0.0.16"}]
+
+        with tempfile.TemporaryDirectory() as tmp:
+            lock_path = Path(tmp) / "smolmachines.lock"
+            results: list[str] = []
+            for _ in range(2):
+                with patch.object(loopback_alias, "_is_macos", return_value=True), \
+                     patch.object(loopback_alias, "_ALLOC_LOCK_PATH", lock_path), \
+                     patch.object(loopback_alias, "_aliases_in_use",
+                                  return_value=in_use_seq.pop(0)):
+                    results.append(loopback_alias.allocate("demo"))
+
+        self.assertEqual(["127.0.0.16", "127.0.0.17"], results)
+
+
 class TestAliasInUseDetection(unittest.TestCase):
     """`_aliases_in_use` inspects every running bundle and pulls
     each container's port-binding `HostIp` out. The detection has