Connections, "every packet" rules, and a packet tracer

- Connections: the kernel's tracked connections, read and closed over
  ctnetlink (internal/conntrack, raw netlink like the drop log), with
  their NAT and TCP state; closing one makes the rules check its next
  packet again (GET /connections, POST /connections/close; hc
  connections; Traffic → Connections).
- Rules can check every packet (every_packet, --every-packet): before
  established connections are accepted, so a drop or reject also stops
  connections already open. They can't accept.
- A packet tracer (internal/trace): a table of its own, inet
  housecarl-trace, marks the packets a filter picks, `nft monitor trace`
  reports every rule and policy they meet in every table, and the tracer
  names what accepted or dropped them, Housecarl's rules by name (POST,
  GET, DELETE /trace; hc trace; Traffic → Trace). One trace at a time,
  at most 200 packets and 20 a second; the table goes when it ends.

e2e: 157 checks, among them an upload a normal block doesn't stop and an
every-packet one does, and a trace naming the blocking rule.

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

README.md

@@ -49,7 +49,9 @@ table of its own.
   rules' order, and keeps what it can't translate as raw rules
   ([docs/import.md](docs/import.md)).
 * **Easy to watch.** Hits per rule, a live log of what was dropped and
-  why, and the history of every change, field by field, with undo.
+  why, the connections the firewall tracks (and closing them), a packet
+  tracer that names the rule that dropped something, in any table, and
+  the history of every change, field by field, with undo.
 * **Its own table only.** Docker, Incus and libvirt keep theirs. The
   confirmed ruleset is loaded at boot, before the network comes up, even
   if housecarld doesn't start.