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/import.md

@@ -159,6 +159,11 @@ Housecarl's table is now in force **alongside** the old one: a packet
 must be accepted by both. Nothing should change for your traffic. If
 Housecarl already has a configuration, `--replace` replaces it.
 
+To be sure first, import in **shadow mode** ([shadow.md](shadow.md)):
+Housecarl's ruleset is then loaded but drops nothing, and its log shows
+each packet the old ruleset lets through that Housecarl would drop. When
+there are none you mind, leave shadow mode and take over.
+
 Imported over the network (not through the local socket), the import is
 on probation like any change: confirm it.