fix(smolmachines): use bridge gateway as TSI proxy host on Linux

On Linux the guest kernel's LOCAL routing table routes all
127.0.0.0/8 to the guest's own loopback interface (priority 0,
checked before any main-table route), so TSI never sees
connections to the per-bottle loopback alias — the fix_guest_
loopback_routing approach confirmed this at the kernel level.

Use the per-bottle docker bridge gateway (192.168.N.1) instead.
It is not a loopback address, so the guest routes it via eth0
and TSI intercepts it normally.  The TSI allowlist remains a
/32 that is distinct from the container IP (192.168.N.2), so
direct bypass to egress:9099 is still blocked by TSI.

Changes:
- Add _proxy_host() helper: returns bundle_gateway on Linux,
  loopback alias on macOS
- Thread proxy_host through _start_bundle, _discover_urls,
  _launch_vm, and _bundle_launch_spec (publish_host_ip)
- Remove _fix_guest_loopback_routing (no longer needed)
- Relax proxy-URL assertion in the integration test to accept
  any http://IP:port (with a comment explaining the difference)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/integration/test_smolmachines_smolvm_smoke.py

@@ -2,7 +2,7 @@
 exercised against the real binary.
 
 The full machine-lifecycle round trip (create → start → exec →
-delete) is gated behind macOS + Darwin platform check and lives
+delete) is gated behind macOS/Linux platform check and lives
 in chunk 2d's smoke. This file just verifies `is_available()`
 correctly reports presence and `_smolvm()` can run a no-op
 subcommand without errors — enough to flag wrapper drift if
@@ -23,8 +23,8 @@ from bot_bottle.backend.smolmachines.smolvm import is_available
     "skipped under act_runner: smolvm not installed on the runner",
 )
 @unittest.skipUnless(
-    platform.system() == "Darwin",
-    "smolvm is macOS-only for v1; Linux+KVM path is a future PRD",
+    platform.system() in ("Darwin", "Linux"),
+    "smolvm requires macOS or Linux",
 )
 @unittest.skipUnless(
     is_available(),