Split workflow-graph.tsx: extract layout, visual maps, and node components

Pure mechanical split, no behavior change (verified: typecheck, lint, knip, 265 unit
tests, build, and the graph-layout e2e all pass). workflow-graph.tsx drops from ~1085
to ~470 lines; Inner + WorkflowGraph stay as the shell and re-export StepStatuses.

New modules under components/workflow-graph/:
- node-data.ts       the NodeData type
- visual-map.tsx     statusTone / integrationBrand / changeRing / changeBadge /
                     issueRing / VERDICT_META, plus StepStatuses + REACHED
- layout.ts          estimateHeight + the dagre layout/centering/straighten pass
- gate-button.tsx    the GateButton (decision + AI reco tooltip)
- step-node.tsx      the StepNode custom node + nodeTypes

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: DylanMerigaud