feat(firecracker): NAT'd egress link for the orchestrator/builder VM

The orchestrator/gateway VM is trusted infra, not an isolated agent: it
builds agent images in-VM (buildah must FROM-pull + apt/npm) and, in the
Stage B cutover, forwards agent egress upstream. Give it a dedicated TAP
(`bborch0`) on a /31 at the top of the IP_BASE /16 (clear of the bbfc*
agent pool at the bottom), NAT'd out the host uplink — while agent VMs
keep their fail-closed, gateway-only isolation table.

- netpool.defaults.env / netpool.py: new BOT_BOTTLE_FC_ORCH_IFACE +
  `orch_slot()` (index -1 sentinel; host x.y.255.0 / guest x.y.255.1).
- scripts/firecracker-netpool.sh: create + address the orchestrator TAP;
  `bot_bottle_fc_nat` table masquerades its /31 out the uplink and
  accepts its forward path. Because bootstrap still runs Docker (whose
  FORWARD policy is DROP), a best-effort, guarded, idempotent DOCKER-USER
  ACCEPT is added too (skipped once Docker is gone). down/status updated.
- nix/firecracker-netpool.nix: mirror the option, pass it via the unit
  Environment= (the store-copied script can't read the defaults file),
  and add iptables to the unit path for the DOCKER-USER step.

Agent isolation is unchanged: the new rules only ever accept/masquerade
the orchestrator link and never drop, so they can't weaken the bbfc*
drops. Applied by re-running `sudo ./scripts/firecracker-netpool.sh up`.

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

bot_bottle/backend/firecracker/netpool.py

@@ -79,6 +79,12 @@ def _cfg(key: str) -> str:
 IFACE_PREFIX = _cfg("BOT_BOTTLE_FC_IFACE_PREFIX")
 NFT_TABLE = _cfg("BOT_BOTTLE_FC_NFT_TABLE")
 
+# The orchestrator/gateway VM's dedicated TAP — outside the bbfc* agent
+# pool and, unlike it, NAT'd to the internet (see `orch_slot`). The
+# orchestrator is trusted infra: it builds agent images in-VM (buildah
+# needs to FROM-pull + apt/npm) and forwards agent egress upstream.
+ORCH_IFACE = _cfg("BOT_BOTTLE_FC_ORCH_IFACE")
+
 
 def pool_size() -> int:
     return int(_cfg("BOT_BOTTLE_FC_POOL_SIZE"))
@@ -123,6 +129,25 @@ def all_slots() -> list[Slot]:
     return [slot(i) for i in range(pool_size())]
 
 
+def orch_slot() -> Slot:
+    """The orchestrator/gateway VM's dedicated link — its own TAP
+    (`ORCH_IFACE`) on a /31 at the TOP of the IP_BASE /16 (host
+    x.y.255.0, guest x.y.255.1), well clear of the agent pool near the
+    bottom of the block. Unlike a pool `Slot`, this link is NAT'd out to
+    the internet by the setup (the orchestrator is trusted infra), so it
+    is deliberately *not* one of the isolated `bbfc*` slots.
+
+    `index` is -1 (sentinel: not a pool index)."""
+    base16 = int(ipaddress.IPv4Address(ip_base())) & 0xFFFF0000
+    host = base16 + 0xFF00
+    return Slot(
+        index=-1,
+        iface=ORCH_IFACE,
+        host_ip=str(ipaddress.IPv4Address(host)),
+        guest_ip=str(ipaddress.IPv4Address(host + 1)),
+    )
+
+
 # --- fail-closed verification ---------------------------------------
 
 def _run_ok(argv: list[str]) -> bool: