feat: Add port-name-padding parameter

Replaces the hard-coded 2pt padding used when placing port name labels.
Configurable per element via port-name-padding on elmt (forwarded by
block and group), defaulting to 2pt to preserve existing layout.

doc/examples.typ

@@ -196,6 +196,7 @@ element.group(
   fill: util.colors.blue.lighten(60%),
   padding: 2em,
   outside-port-names: true,
+  port-name-padding: 4pt,
   ports: (
     west:  ((id: "in",  name: "D"),),
     east:  ((id: "out", name: "Q"),),
@@ -209,12 +210,14 @@ wire.wire("wG1in",  ("g1-port-in", "b1-port-d"), directed: true)
 wire.wire("wG1out", ("b1-port-q",  "g1-port-out"), directed: true)
 
 // outside-port-names: false — labels inside the border (block style)
+// with extra padding via port-name-padding
 element.group(
   id: "g2", name: "inside",
   stroke: (dash: "dashed"),
   fill: util.colors.orange.lighten(60%),
   padding: 2em,
   outside-port-names: false,
+  port-name-padding: 0.4em,
   ports: (
     west:  ((id: "in",  name: "D"),),
     east:  ((id: "out", name: "Q"),),