Shadow mode: Housecarl beside the firewall in force, changing nothing

shadow.enabled in housecarl.yaml loads Housecarl's ruleset so that it
blocks, rejects and translates nothing: drops and rejects count and
accept (an accept only ends Housecarl's chain), policies accept and log
what they would drop, NAT only counts, MSS clamping and the flowtable
are left out, and raw rules that do more than accept, drop or reject are
left out with a warning. Comments mark what would happen ([would drop]).
Filter chains run after the other firewall's (+10) or before (-10); NAT
chains always first. Every change is checked by nftables in both forms.
No probation; takeovers and closing connections are refused. The boot
fallback filters nothing. internal/tablewatch puts the table back after
a flush ruleset. Mode in status, healthz, whoami, the web UI, the log
and the tracer; hc ruleset --enforcing. docs/shadow.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -49,6 +49,10 @@ table of its own.
   ruleset into zones, sets, services, rules and port forwards, keeping the
   rules' order, and keeps what it can't translate as raw rules
   ([docs/import.md](docs/import.md)).
+* **Try it first, in shadow mode.** Housecarl's ruleset runs beside the
+  firewall you have, blocking nothing: its hits, log and tracer show what
+  it would drop, so you see what switching over would break before you do
+  ([docs/shadow.md](docs/shadow.md)).
 * **Easy to watch.** A zone matrix of what is allowed between zones,
   statistics over time (traffic per zone and device, drops, the sources
   blocked most, connections; [docs/stats.md](docs/stats.md)), hits per rule, a live log of what was dropped and
@@ -104,6 +108,7 @@ The web UI is on `http://127.0.0.1:8070/` (see `listen` in
 * [rules.md](docs/rules.md): zones, address sets, services, rules and the built-ins
 * [nat.md](docs/nat.md): port forwards, hairpin NAT, IPv6 pinholes, source NAT, redirects
 * [import.md](docs/import.md): moving from an existing nftables ruleset
+* [shadow.md](docs/shadow.md): shadow mode, Housecarl beside your firewall, changing nothing
 * [operations.md](docs/operations.md): probation, the boot and its fallback, hits and the log, monitoring, backups, troubleshooting
 * [flowtable.md](docs/flowtable.md): the fast path for forwarded traffic, and when (not) to use it
 * [certificates.md](docs/certificates.md): HTTPS with a certificate from Let's Encrypt, through Jarl