feat: Support cross-element port alignment via align-with

Extend the port `align-with` field to accept references to ports of
other elements, in addition to the existing local-port form:

  - "<local-id>"                    → same-element port (unchanged)
  - (element: "<id>", port: "<id>") → cross-element reference
  - "<raw-anchor>"                  → arbitrary CeTZ anchor name

Cross-element alignment is implemented via CeTZ perpendicular
projection (`|-`) so the port sits on this element's side line while
sharing the remote port's y (east/west) or x (north/south) coordinate.
Local resolution still validates matching axis and detects cycles;
external refs skip axis validation (remote side unknown at eval time)
and require the target element to be drawn first.

Also enrich port-related panic messages with the element id and port
context to make offending figures easier to locate, and add a manual
example (`examples.align-with-ports`) demonstrating both local and
cross-element alignment.

doc/examples.typ

@@ -23,6 +23,31 @@ wire.stub("block-port-in2", "west")
 wire.stub("block-port-out", "east")
 ```)
 
+#let align-with-ports = example(```
+// `align-with` reuses another port's coord
+// (E/W: y, N/S: x). Forms:
+//  1. "id"                     — local
+//  2. (element:"e", port:"p")  — cross-elmt
+//  3. "raw-anchor"             — CeTZ anchor
+element.block(x: 0, y: 0, w: 2, h: 2, id: "b1",
+  ports: (
+    west: ((id: "a", name: "A", y: 30%),),
+    east: ((id: "oa", name: "A'",
+            "align-with": "a"),),
+  ))
+element.block(x: 4, y: -.3, w: 2, h: 2.6, id: "b2",
+  ports: (
+    west: ((id: "ia", name: "A",
+            "align-with":
+              (element: "b1", port: "oa")),),
+    east: ((id: "o", name: "C"),),
+  ))
+wire.stub("b1-port-a", "west")
+wire.wire("w", ("b1-port-oa", "b2-port-ia"),
+          style: "direct")
+wire.stub("b2-port-o", "east")
+```, vertical: true)
+
 #let extender = example(```
 element.extender(
   x: 0, y: 0, w: 3, h: 1,