feat(firecracker): move pool off CGNAT, add overlap guard + flake module

The default TAP-pool base was 100.64.0.0/10 (RFC-6598 CGNAT) — chosen to
dodge RFC-1918, but that's exactly the range Tailscale assigns node
addresses from, so on a Tailscale host it's the worst pick. Move the
default to 10.243.0.0/16, an obscure RFC-1918 block that steers clear of
docker/libvirt/k8s/LAN and Tailscale.

No default is collision-proof, so add netpool.overlapping_routes(): it
parses `ip -json route show table all` and flags any route intersecting
the pool range (excluding our own bbfc* TAPs and the default route). The
launch preflight warns on overlap; `backend status` reports it.

Distribute the NixOS host setup as a flake module instead of a
copy-pasted blob: nix/firecracker-netpool.nix computes the taps / nft
table from typed options (poolSize, ipBase, ifacePrefix, owner) with a
/31-alignment assertion, and flake.nix exposes it as
nixosModules.firecracker-netpool. Defaults mirror the backend constants;
writeEnvFile emits the matching BOT_BOTTLE_FC_* so the host pool and the
launcher can't drift.

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

bot_bottle/backend/firecracker/util.py

@@ -9,7 +9,7 @@ generation.
 
 The privileged network setup (TAP pool + nft table) is a one-time
 operator step — see `netpool.py`, `scripts/firecracker-netpool.sh`,
-and `./cli.py firecracker setup`.
+and `./cli.py backend setup --backend=firecracker`.
 """
 
 from __future__ import annotations
@@ -124,17 +124,25 @@ def _require_network_pool() -> None:
     empirical isolation probe run after boot, before the agent starts
     (see isolation_probe.verify_isolation). What we must never do is
     boot without the TAP pool."""
+    conflicts = netpool.overlapping_routes()
+    if conflicts:
+        detail = "; ".join(f"{c.dst} dev {c.dev}" for c in conflicts)
+        warn(f"Firecracker pool range ({netpool.ip_base()}, "
+             f"{netpool.pool_size()} slots) overlaps existing routes: "
+             f"{detail}. This can shadow or be shadowed by that route; "
+             f"set BOT_BOTTLE_FC_IP_BASE to a free range and re-run "
+             f"./cli.py backend setup --backend=firecracker.")
     missing = netpool.missing_taps()
     if missing:
         die(f"network pool incomplete — missing TAP devices: "
-            f"{', '.join(missing)}.\n    ./cli.py firecracker setup")
+            f"{', '.join(missing)}.\n    ./cli.py backend setup --backend=firecracker")
     if shutil.which("nft") is not None and not netpool.nft_table_present():
         # nft is queryable and says the table is absent — that's a
         # definite, catchable misconfiguration; fail early.
         warn(f"isolation table `inet {netpool.NFT_TABLE}` not found via nft. "
              "If this is a permissions issue it will be re-checked "
              "empirically after boot; otherwise run: "
-             "./cli.py firecracker setup")
+             "./cli.py backend setup --backend=firecracker")
 
 
 # --- rootfs pipeline (rootless) -------------------------------------