Add fast iteration tooling

nix run .#fast, .#test and .#webui-dev; see DEVELOPING.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

DEVELOPING.md

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+# Developing
+
+Three tools, from instant to slow. Run them from the repository root; the
+flake only sees files git knows about, so `git add` new files first.
+
+## `nix run .#webui-dev`: the web UI on your machine
+
+Sample settings, sample DHCP leases and your real interface counters, on
+<https://127.0.0.1:8443> (password `devpassword1`). Templates and CSS are
+re-read on every request: edit, reload the browser. A template with a typo
+keeps the last good version and logs the error. Pings need root and show "no
+answer"; Apply does nothing useful here.
+
+## `nix run .#fast`: no VMs
+
+Go formatting, vet and tests, then builds the installer, a router built from
+settings (through the release) and the home gateway. That runs shellcheck over
+the scripts and evaluates the modules: most failures of the VM tests
+(typos, type errors, a script that does not build) show up here, in about a
+minute (half of that when little changed).
+
+    nix run .#fast -- --drivers   also builds every VM test's driver: type-checks
+                                  the test scripts, builds the VMs' systems
+    nix run .#fast -- --all       also the checks that need no VM and `nix flake check`
+
+## `nix run .#test -- NAME...`: VM tests
+
+One at a time (each needs gigabytes of memory), a line per test with its time,
+and for a failure the step that failed and what it said; the full log is in
+`${TMPDIR:-/tmp}/router-test-logs/NAME.log`.
+
+    nix run .#test -- update setup
+    nix run .#test -- --all
+    nix run .#test -- --list
+
+`nix run .#test -- -i update` builds the test's interactive driver and gives a
+Python prompt: `start_all()`, `target.succeed("...")`, `test_script()`. Boot
+the VMs once and try a step again instead of running the whole test.
+
+### Where the time goes
+
+A VM test spends most of its time in the system under test, not in building:
+`router-install` (about a minute), booting (20 s each), and everything that
+runs `router-apply` on the router (building a NixOS system there takes about a
+minute) or waits for a rollback timer. A source change rebuilds the systems
+again on the host. So:
+
+- Try a change with `webui-dev` or `fast` first; run a VM test only to confirm.
+- Run only the test for what you changed: `update` for `router-update` and
+  the Update page, `setup` for `router-setup`, `apply` for the settings and
+  the web UI, `basic` for the firewall and DHCP.
+- Use `-i` to iterate on a test script.