Migrate dependency & Diagram graphs to nx.MultiDiGraph (retire alias-node workaround)

[dimitri-yatsenko]

Summary

Migrate Dependencies and Diagram (both currently nx.DiGraph subclasses) to
nx.MultiDiGraph, retiring the integer "alias node" workaround used to represent
multiple / renamed foreign keys between the same pair of tables.

Background

A plain nx.DiGraph permits only one edge per ordered node pair, so it cannot
represent two foreign keys between the same two tables. To work around this,
dj.Diagram inserts an integer-named alias node that splits a renamed FK into
two half-edges (parent → "N" → child), giving each FK its own edge identity
(dependencies.py:216–223).

dj.Diagram predates NetworkX's multigraph support in our design (the class dates
to ~2016). NetworkX has long shipped MultiDiGraph (parallel edges keyed by
(u, v, key)), so the alias-node workaround is no longer necessary — it is a
historical choice, not a NetworkX limitation.

Scope — what this removes

Migrating to MultiDiGraph deletes the alias machinery:

• ~14 .isdigit() special-case checks in diagram.py
• _find_real_edge_props (diagram.py)
• alias-follow branches in both _propagate_restrictions and _propagate_restrictions_upstream
• alias-node insertion at dependencies.py:220–223
• alias-collapse in topo_sort (dependencies.py:62–73)

Traversals we rely on (ancestors, descendants, topological_sort,
shortest_path, node_boundary) all work on multigraphs. The main change is that
edge-data access moves to (u, v, key) addressing throughout.

Diagramming notation implications

Alias nodes currently double as the visual marker for renamed FKs. Removing them
means we need new notation:

• Renamed-FK edge notation. Design notation for a renamed foreign key as an
  edge attribute (e.g. a labeled / styled / dashed edge showing child_col ← parent_col)
  rather than an intermediate node.

• Multigraph rendering support. Ensure the render backends handle parallel edges
  between the same node pair:

  • pydot / pygraphviz → dot / graphviz (make_dot, make_svg, to_pydot at diagram.py:1540)
  • Mermaid (make_mermaid at diagram.py:1642)
  • matplotlib draw

  Graphviz/dot support parallel edges natively; Mermaid flowcharts need multiple edge
  statements between the same nodes — verify edge labels don't collide.

Risk

Broad, cross-cutting, and test-heavy: every edge-data access site plus the full
cascade / trace / restrict suites must be re-verified under multigraph edge semantics.
Not a patch-release change — hence v2.4.

References

• Surfaced during the 2.3.1 dj.Diagram review (alias-node design rationale).
• Code: src/datajoint/dependencies.py, src/datajoint/diagram.py.