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>

CHANGELOG.md

@@ -70,6 +70,12 @@ the way Jarl and Steward are.
 - **Warnings about the machine:** while `nftables.service` is enabled
   (it loads `/etc/nftables.conf` at boot, and flushes every table when
   stopped), and while the table an import came from still applies.
+- **Connections and tracing:** the connections the kernel tracks, with
+  their NAT, and closing them so the rules check them again (Traffic →
+  Connections, `hc connections`); a rule option, "every packet", for
+  blocks that stop connections already open; and a packet tracer
+  (Traffic → Trace, `hc trace`) that follows packets through every
+  table's rules and names what accepted or dropped them.
 - **Hits and the log:** packet and byte counters for every rule, NAT rule,
   built-in and policy; packets logged by rules and policies, read from
   netlink log group 4171 (`hc log -f`, the Log page).