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>

internal/server/server.go

@@ -67,6 +67,8 @@ type Firewall interface {
 	SaveBoot(st *model.State) error
 	// ResetQuota starts a rule's quota again (perAddress: its sets).
 	ResetQuota(ctx context.Context, ruleID int64, perAddress bool) error
+	// Trace follows packets (internal/trace) until ctx ends.
+	Trace(ctx context.Context, script string, line func(string)) error
 }
 
 // Server is the HTTP server.
@@ -91,6 +93,10 @@ type Server struct {
 	// unitLinks are where an enabled nftables.service shows (tests change
 	// them).
 	unitLinks []string
+	// conns is the kernel's connection tracking (nil: unavailable).
+	conns Conntrack
+	// tracing is the packet trace running (or the last one).
+	tracing tracing
 	// csMu orders changes to changesets (staging, applying, discarding).
 	csMu  sync.Mutex
 	nowFn func() time.Time