feat: add shell parameter and outside port names to block

Add `port-names-outside` to draw port name labels outside the block
border. It defaults to `auto`, which follows the new `shell` setting.

Add `shell` to `block` so it can act as a container, as an alternative
to `group`. Placing elements into such a block lets them be aligned to
the ports of the outer block. A shell block draws its port names outside
and, through the existing `wire-inside` behaviour, attaches wires to the
inner tip of pentagon ports. Child elements can be passed as a
positional body argument.

Add the `port-names-outside` and `shell` examples.

doc/examples.typ

@@ -228,6 +228,20 @@ wire.stub("g2-port-in",  "west")
 wire.stub("g2-port-out", "east")
 ```, vertical: true)
 
+#let port-names-outside = example(```
+element.block(
+  x: 0, y: 0, w: 2, h: 2, id: "block",
+  port-names-outside: true,
+  port-name-padding: 4pt,
+  ports: (
+    north: ((id: "clk", name: "CLK", clock: true),),
+    west: ((id: "in1", name: "A"),
+           (id: "in2", name: "B")),
+    east: ((id: "out", name: "Y"),)
+  )
+)
+```)
+
 #let port-shape = example(```
 // Block with pentagon-shaped ports.
 // - port-shape / port-kind / port-size are element-level defaults.
@@ -260,6 +274,29 @@ wire.stub("b-port-clk", "north")
 wire.stub("b-port-rdy", "south")
 ```)
 
+#let shell = example(```
+// A shell block acts as a container: port name labels are drawn
+// outside by default, and wires attach at the *inside* tip of
+// pentagon-shaped ports so child elements can sit within.
+element.block(
+  id: "sh", w: 5, h: 3, x: 0, y: 0,
+  shell: true,
+  port-shape: "pentagon",
+  fill: util.colors.blue.lighten(80%),
+  ports: (
+    west: ((id: "in",  name: "IN"),),
+    east: ((id: "out", name: "OUT", kind: "out"),),
+  ),
+  {
+    element.block(id: "inner", w: 2, h: 1.5, x: 1.5, y: 0.75,
+      fill: util.colors.yellow,
+      ports: (west: ((id: "a"),), east: ((id: "b"),)))
+  }
+)
+wire.wire("w-in",  ("sh-port-in",  "inner-port-a"))
+wire.wire("w-out", ("inner-port-b", "sh-port-out"))
+```)
+
 #let intersection = example(```
 wire.wire("w1", ((0, 0), (1, 1)), style: "zigzag")
 wire.wire("w2", ((0, 0), (1, -.5)),