Fix overlapping parallel edges between same node pair

[cmwright]

Problem

Parallel edges between the same (from, to) node pair were drawn on top of each other. In the reported vendor-onboarding flowchart, the two Keep waiting → Analyst review approval gate edges (submitted / window lapses) overlapped completely, where mermaid.js splits them into two separate curves.

Root cause

Every edge was registered with dagre under a None name, so parallel edges collapsed to a single edge ID in the multigraph — the second overwrote the first. Downstream extraction and routing maps were keyed only by (from, to), so both edges shared one routing path and one label position.

Fix

Thread a per-edge identity (the edge's positional index — stable because all_edges is iterated in identical order by both graph_builder and route_edges) through the pipeline:

• graph_builder: assign each edge a unique dagre name so the multigraph + normalize step give each parallel edge its own dummy chain (distinct routing and label-dummy positions).
• types: add EdgeKey = (String, String, Option<String>).
• mod: key DagreEdgeExtraction maps by EdgeKey; subgraph-fallback cleanup retains by node pair regardless of name.
• edge_routing: reconstruct the matching key when routing.

Edge names survive dagre's acyclic reversal (restored via forward_name on undo), so keys stay consistent after layout.

Testing

• Added test_parallel_edges_are_separated asserting the two parallel wait2 → review edges get distinct label positions and distinct routed paths.
• Rendered the reported diagram — the two edges now curve apart with side-by-side labels, matching mermaid.js.
• Full workspace test suite green (0 failures).

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