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".

cli/parse_test.go

@@ -22,7 +22,7 @@ func TestParseArgs(t *testing.T) {
 		{"permute flag after uri", []string{"u", "--split=8"}, Run, map[string]string{"split": "8"}, []string{"u"}},
 		{"permute interleaved", []string{"a", "-c", "b", "--quiet"}, Run, map[string]string{"continue": "true", "quiet": "true"}, []string{"a", "b"}},
 		{"dash dash terminator", []string{"--split=8", "--", "--not-a-flag", "u"}, Run, map[string]string{"split": "8"}, []string{"--not-a-flag", "u"}},
-		{"bt metadata timeout", []string{"--bt-metadata-timeout=120", "magnet:x"}, Run, map[string]string{"bt-metadata-timeout": "120"}, []string{"magnet:x"}},
+		{"long int + magnet uri", []string{"--bt-stop-timeout=120", "magnet:x"}, Run, map[string]string{"bt-stop-timeout": "120"}, []string{"magnet:x"}},
 		{"help", []string{"--help"}, ShowHelp, nil, nil},
 		{"help short", []string{"-h"}, ShowHelp, nil, nil},
 		{"version", []string{"-v"}, ShowVersion, nil, nil},
@@ -68,21 +68,6 @@ func TestParseArgsErrors(t *testing.T) {
 	}
 }
 
-// got bounds the magnet metadata fetch by default rather than waiting forever;
-// guard the default value and its zero floor.
-func TestBTMetadataTimeoutDefault(t *testing.T) {
-	o, ok := byLong["bt-metadata-timeout"]
-	if !ok {
-		t.Fatal("bt-metadata-timeout option is missing")
-	}
-	if o.Default != "60" {
-		t.Errorf("bt-metadata-timeout default = %q, want %q", o.Default, "60")
-	}
-	if o.Min != 0 {
-		t.Errorf("bt-metadata-timeout Min = %d, want 0 (reject negatives, allow 0=off)", o.Min)
-	}
-}
-
 func TestParseSize(t *testing.T) {
 	tests := []struct {
 		in   string