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.
David Stadelmann
fff5b2ecd5