cli: trim basic help, drop invented flag, parallelize by default

Surface area, not the machinery, was the problem: 16 flags crowded the
bare --help. Re-tag so basic shows only the 10 outcome-changing flags;
every other flag still works behind --help=<group>.

- cut --bt-metadata-timeout, an invented flag with no real counterpart. Keep
  its 60s magnet-metadata stall as a fixed internal default (bt.go), not a
  user knob.
- fix the parallelism footgun: a bare `got URL` used min(split,x)=1
  connection. When -x is unset, let --split drive per-host connections
  (capped at 16), so `got URL` gets 5 and `-s16` gets 16. An explicit -x
  is honored verbatim, including -x1. Locked by TestHTTPConnDefault.
- -s is the per-download speed dial now, so it moves to basic; -x (a
  per-server cap) moves to the http group. -j help says "files at once,
  not connections within one file".

main_test.go

@@ -144,6 +144,36 @@ func TestGatherURIsGrouping(t *testing.T) {
 	}
 }
 
+// TestHTTPConnDefault locks the parallelism default: a bare `got URL` must use
+// several connections, not one. With -x unset, --split drives the per-host
+// connection count (capped at the -x ceiling of 16); an explicit -x is honored
+// verbatim, including the aria2-faithful -x1 single connection.
+func TestHTTPConnDefault(t *testing.T) {
+	cfg := func(args ...string) httpdl.Config {
+		res, err := cli.Parse(append([]string{"--no-conf"}, args...))
+		if err != nil {
+			t.Fatalf("parse %v: %v", args, err)
+		}
+		return httpConfig(res.Options, true, nil)
+	}
+	tests := []struct {
+		name string
+		args []string
+		want int
+	}{
+		{"bare URL uses split default", []string{"http://a/x"}, 5},
+		{"-s16 means 16", []string{"-s16", "http://a/x"}, 16},
+		{"-s100 capped at 16 per host", []string{"-s100", "http://a/x"}, 16},
+		{"explicit -x4 honored", []string{"-x4", "http://a/x"}, 4},
+		{"explicit -x1 stays single", []string{"-x1", "http://a/x"}, 1},
+	}
+	for _, tc := range tests {
+		if c := cfg(tc.args...); c.MaxConnPerServer != tc.want {
+			t.Errorf("%s: MaxConnPerServer = %d, want %d", tc.name, c.MaxConnPerServer, tc.want)
+		}
+	}
+}
+
 // TestReadInputFileTAB checks the input-file grouping: TAB-separated URIs on
 // a line are mirrors of one download; a plain line is one download.
 func TestReadInputFileTAB(t *testing.T) {