Feature request: bus / vector wires — render one edge for a contiguous port slice

Bắt đầu bằng cách lần theo việc kết xuất edge hiện có, hành vi kéo port-handle và cách xử lý nhiều edge trong frontend PathView. Xác định cách các slice liên tiếp, nhãn, tương tác thu gọn/mở rộng và thao tác kéo nhóm nên phù hợp với view model hiện tại mà không thay đổi graph .pvm, các đối tượng Connection hoặc Python được export. Hoàn tất khi các edge bus vẫn chỉ dùng để xem, có thể được mở rộng và thu gọn lại, đồng thời giữ nguyên đầu ra graph hiện có.

Description

Allow a set of edges connecting a contiguous port slice between two blocks to be rendered and
manipulated as a single bus edge:

• Collapsed by default when a drag connects a slice, or when N parallel edges between the same
  block pair are detected — drawn as one thicker line, labelled with the slice (e.g. species[0:10]
  or a user-supplied name like exhaust).
• Expandable on demand — click to fan out into the individual edges for inspection or to rewire
  one channel, then re-collapse.
• Purely a view concern. The underlying .pvm graph, the Connection objects, and the exported
  Python would be unchanged — a bus is a grouping annotation over existing edges, not a new
  connection type. That keeps pathview convert output identical and means no PathSim-side change
  is needed.

Two smaller things that would compose well with it, though they're separable:

• Drag from a port group (shift-drag, or drag from a block edge rather than a handle) to create the
  whole slice in one gesture, instead of ten drags.
• A bus label on the edge — this overlaps with #8 (Add labels/annotations to Edges), so the two may
  want to share an implementation.

Problem

PathSim handles multi-variable signals well via port slices — Connection(a[0:10], b[0:10]) is
concise in Python and works exactly as you'd want. In PathView the same connection is 10 separate
edges, because each port is an individual handle.

For anything with a chemical or multi-species signal this scales badly. We're building a fusion
fuel-cycle and balance-of-plant block library on PathSim, where every stream carries a molar-flow vector over a species basis, typically 10 species (H2, HD, HT, D2, DT, T2, He3, He4, N2, NT3), and users can extend the basis at runtime.
A ~25-block fuel cycle therefore draws roughly 250 edges for what a person reading the diagram
thinks of as ~25 pipes.

The Python model is fine. It's the diagram that stops being readable, which is a shame because the
diagram is the reason to use PathView at all.

This isn't fusion-specific — it applies to any chemistry, thermofluid, or power-system model where a
"signal" is naturally a vector.

Alternatives Considered

No response

Additional Context

No response