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.defaults.env

@@ -15,3 +15,11 @@ BOT_BOTTLE_FC_POOL_SIZE=8
 BOT_BOTTLE_FC_IP_BASE=10.243.0.0
 BOT_BOTTLE_FC_IFACE_PREFIX=bbfc
 BOT_BOTTLE_FC_NFT_TABLE=bot_bottle_fc
+# The orchestrator/gateway VM's own TAP — a dedicated link OUTSIDE the
+# bbfc* agent pool. Unlike agent VMs (which reach only their gateway),
+# the orchestrator is trusted infra that needs real NAT'd internet
+# egress: to FROM-pull + apt/npm during in-VM agent-image builds
+# (buildah) and to forward agent egress upstream (Stage B gateway). Its
+# /31 is the top of the IP_BASE /16 (host x.y.255.0, guest x.y.255.1),
+# clear of the pool near the bottom of the block.
+BOT_BOTTLE_FC_ORCH_IFACE=bborch0