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>

docs/nat.md

@@ -7,6 +7,10 @@ public port are sent to the host, and let through the forward chain.
 One object does what takes three rules by hand (the DNAT, the forward
 accept, and the hairpin masquerade).
 
+In shadow mode ([shadow.md](shadow.md)) NAT rules translate nothing:
+they count the connections they would translate, and the other
+firewall's NAT does the work.
+
 ```sh
 hc forward add 8443 --to 192.168.1.10:443 --public 203.0.113.10 --hairpin --name web
 hc forward add 27015-27030 --proto tcp,udp --to 192.168.1.20 --public 203.0.113.10 --hairpin --name game