feat(orchestrator): slice 13c(ii) — shared-gateway source-IP allocator

Deterministic per-bottle address assignment on the consolidated docker
gateway's shared network — the address the gateway uses as its attribution
key. Pure ipaddress logic; the docker-specific inputs (subnet CIDR, the
gateway container's own address) are gathered by the caller and passed as
`taken`, keeping it testable without docker.

- next_free_ip(cidr, taken): lowest host address not reserved (network +
  broadcast excluded by hosts(); docker's router .1 excluded here) and not
  already assigned (gateway container + live bottles from the registry).
  NoFreeAddressError when the subnet is exhausted.

pyright 0 errors; pylint 9.83/10; unit suite green (the 13 test_sidecar_init
/bin/sleep errors are pre-existing NixOS-local noise).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

tests/unit/test_gateway_net.py

@@ -0,0 +1,42 @@
+"""Unit: shared-gateway source-IP allocation (PRD 0070)."""
+
+from __future__ import annotations
+
+import unittest
+
+from bot_bottle.backend.docker.gateway_net import NoFreeAddressError, next_free_ip
+
+
+class TestNextFreeIp(unittest.TestCase):
+    def test_skips_router_and_returns_first_host(self) -> None:
+        # .1 is docker's router; the first assignable address is .2.
+        self.assertEqual("172.18.0.2", next_free_ip("172.18.0.0/16", []))
+
+    def test_skips_taken_addresses(self) -> None:
+        # Gateway container holds .2, a live bottle holds .3 -> next is .4.
+        self.assertEqual(
+            "172.18.0.4", next_free_ip("172.18.0.0/16", ["172.18.0.2", "172.18.0.3"]),
+        )
+
+    def test_taken_order_does_not_matter(self) -> None:
+        self.assertEqual(
+            "172.18.0.2", next_free_ip("172.18.0.0/16", ["172.18.0.3", "172.18.0.5"]),
+        )
+
+    def test_allocation_is_deterministic(self) -> None:
+        taken = ["172.18.0.2"]
+        self.assertEqual(next_free_ip("172.18.0.0/16", taken),
+                         next_free_ip("172.18.0.0/16", taken))
+
+    def test_raises_when_subnet_exhausted(self) -> None:
+        # /30: hosts are .1 (router, reserved) and .2; taking .2 leaves none.
+        with self.assertRaises(NoFreeAddressError):
+            next_free_ip("10.9.9.0/30", ["10.9.9.2"])
+
+    def test_accepts_host_bits_set_cidr(self) -> None:
+        # A container's inspected address arrives as e.g. 172.18.0.2/16.
+        self.assertEqual("172.18.0.2", next_free_ip("172.18.0.5/16", []))
+
+
+if __name__ == "__main__":
+    unittest.main()