refactor(firecracker): single-source the network-pool defaults

The pool params (size, IP base, iface prefix, nft table) were triplicated
— hardcoded in netpool.py, scripts/firecracker-netpool.sh, and
nix/firecracker-netpool.nix — plus the IP math (3x) and the nft ruleset
(2x). Nothing enforced agreement; changing the base (ce3fad9, off CGNAT)
forced a coordinated three-file edit, and a missed one would silently
provision a range the launcher doesn't expect.

Collapse to one source of truth:

  * netpool.defaults.env — a plain KEY=VALUE file (bash-sourceable,
    systemd EnvironmentFile-compatible, Python- and Nix-parseable) holding
    the four defaults. A BOT_BOTTLE_FC_* env var still overrides any key.
  * netpool.py reads it for the Python defaults (missing file = hard
    error, not confusing empty defaults).
  * the shell script falls back to it (no literal `:-8` / `10.243.0.0`),
    and its `up` is now non-destructive/idempotent (only creates a
    missing TAP), so re-running never cuts a live VM.
  * the Nix module readFile-parses it for its option defaults and
    delegates bring-up to the SAME shell script (dropping its duplicate
    IP math, nft ruleset, and TAP loop) — passing every value as
    Environment= so the store-detached script never needs the file.

Net: defaults 3x -> 1x, nft ruleset 2x -> 1x, TAP loop 2x -> 1x. The one
remaining IP-math dup (Python launch-addressing vs bash bring-up) is
justified — different runtimes. Tests now guard the invariant (Python
reads the shared file; the script/module hold no literals) instead of
pinning duplicated strings.

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

README.md

@@ -5,7 +5,7 @@
 # bot-bottle
 
 [![test](https://gitea.dideric.is/didericis/bot-bottle/actions/workflows/test.yml/badge.svg?branch=main)](https://gitea.dideric.is/didericis/bot-bottle/actions?workflow=test.yml)
-[![coverage](https://img.shields.io/badge/coverage-84%25-brightgreen)](https://coverage.readthedocs.io/)
+[![coverage](https://img.shields.io/badge/coverage-82%25-brightgreen)](https://coverage.readthedocs.io/)
 [![core coverage](https://img.shields.io/badge/core%20coverage-95%25-brightgreen)](https://gitea.dideric.is/didericis/bot-bottle/src/branch/main/docs/decisions/0004-coverage-policy.md)
 
 **Problem:** Developer wants to run a coding agent without supervision, but they don't want a prompt injected or misbehaving agent wrecking their environment or exfiltrating sensitive data.