Edge label positioning

[Wagner-Laranjeiras]

Fix #514: respect external positions for edge labels

Summary

When using the ELK layout integration, edge labels were rendered at the position
computed by Sprotty's EdgeLayoutPostprocessor instead of the position chosen by
ELK. This PR introduces a small cross-package marker that lets a layout engine
declare "this label's position is final, please don't recompute it," and routes
the rendering pipeline accordingly.

Fixes #514.

Changes

• sprotty-protocol — adds a new ExternallyPositioned interface so layout
  engines and the framework can speak about externally-set positions.
• sprotty-elk — applyEdge now marks every edge label it positions with
  externallyPositioned: true.
• sprotty — EdgeLayoutPostprocessor skips externally-positioned labels;
  LocationPostprocessor (the default position applier) is updated to handle
  them instead, so they still get a transform reflecting ELK's coordinates.
• A new checkExternallyPositioned type guard in features/edge-layout/model.ts
  mirrors the existing checkEdgePlacement pattern.

The fix is opt-in: only labels positioned by an external engine are affected.
Existing diagrams that rely on EdgeLayoutPostprocessor's default behavior are
unchanged.

Testing

• New unit test in packages/sprotty-elk/src/elk-layout.spec.ts verifies that
  the ELK adapter marks edge labels as externallyPositioned.
• Existing tests in the same suite continue to pass.
• Manually verified in the random-graph example with NODES = 2, EDGES = 1
  and an edge label, mirroring the issue's reproduction. With the fix applied,
  the label appears at ELK's chosen position; reverting the marker assignment
  reproduces the original bug.

[gfontorbe]

Hi @Wagner-Laranjeiras , thanks for the contribution.

This is a good start. However the approach completely bypasses the post-processing if the label has been placed by ELK. This create the issue that rotation and bounding to the edge are not preserved (making every label "free").

Ideally this should also be layout engine agnostic and not depend on sprotty-elk