feat(flow): BPMN-flavored swimlanes — pools + nested lanes + message-flow-crosses-pool lint

[ivanmkc]

Gap

Swimlane support today is dagre lanes:true (flat lanes). Real BPMN/process workflows need more (Camunda, ProcessMind pools & lanes):

• Pool → lane → sub-lane nesting (not just one flat level).
• Horizontal and vertical orientation.
• Message-flow vs. sequence-flow distinction (dashed + open-circle vs. solid) — depends on the edge-marker work.
• The constraint that sequence flows cannot cross pool boundaries — lintable.

Proposal

• Extend the grouped/lanes model to a pool→lane hierarchy + orientation switch.
• Add a flow-geometry lint: warn when a sequence-flow edge crosses a pool boundary.
• (BPMN gateways + the ~30 event glyphs are a separate, larger effort — track only if a BPMN audience is explicitly targeted.)

Surface / effort

Mostly dagre layout + a lint; message-flow styling depends on the edge-marker issue. M.

Part of the external-flowchart parity audit (see tracking epic).