feat: sankey diagrams — branching flow ribbons

[ethenotethan]

What

The other half of the flow-shaped gap from the chart-type audit — waterfall (#196) covered sequential where-did-it-go; sankey covers branching flows: budget allocation, token/traffic routing, funnels.

```sankey
{"title": "Q2 Budget", "links": [
  {"from": "Revenue", "to": "Engineering", "value": 400},
  {"from": "Revenue", "to": "Marketing", "value": 150},
  {"from": "Engineering", "to": "Salaries", "value": 320}
]}
```

• Nodes implicit from links — the lowest-friction contract for a model to emit. Optional nodes array adds groups for coloring. Self-loops and non-positive values drop at parse.
• Pure layout engine (SankeyLayout, unit-tested): cycle-safe longest-path column assignment (bounded passes — a cyclic link renders adjacent instead of hanging), node bars sized by throughput on a global scale so heights compare across columns, ribbons stacked without overlap down each node edge.
• Rendering: Canvas cubic ribbons (thickness ∝ value) under node bars, native-text labels (last column leans left). Click a node → its flows light and a summary shows in/out totals per link, with an unbalanced-flow Δ warning when outflow ≠ inflow.
• Routes as a chat fence, captures as a living artifact (renders in the Artifacts pane + iOS sheet), prompt documents the contract. PDF-safe by construction (Canvas + text, no representables).
• No collision with mermaid's sankey (that arrives as ```mermaid/sankey-beta and keeps its route).

Testing

3 suites-worth: parse hygiene (self-loops/zero/negative dropped), layout invariants (topological columns, global-scale proportional spans, no-overlap ribbon stacking), cycle termination. Both targets build, swiftlint --strict clean.

Built in an isolated worktree off main.

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