feat(flow): tighten grouped layout rhythm

[ivanmkc]

Summary

• centralize an 8px grouped-layout rhythm with a derived 56px header/member inset
• give architecture tiers one measured width and compact sequential zones to a 64px routing channel
• pack swimlane dependency ranks from measured per-lane extents with a guaranteed 40px channel
• define explicit line-height hierarchy for change nodes, entity nodes, group labels, and edge labels
• derive painted-DOM verifier thresholds from the shared spacing contract

This PR is stacked on #308. Tracks #154.

Verification

• viewer unit suite: 790/790 passed
• production viewer build passed
• strict overlap corpus: 32/32 clean, 0 known-bad, 0 regressed
• painted tiers, zones, and swimlanes: no overlaps, spillout, anchor mismatch, or edges over nodes
• LR, RL, TB, and BT branch/rejoin rank-channel tests passed
• independent code review: no Critical or Important findings

Before and after

Mode	Before	After
Architecture tiers
Data-lineage zones
Swimlanes opening view

Full swimlane topology