Construct grouped tiers from measured geometry

[ivanmkc]

Summary

• measure painted node, group-label, and edge-label bounds before visible layout
• construct tier containers with a shared 8px-grid spacing contract (24px container padding, 32px member gaps, 16px/12px label inset)
• route fixed orthogonal edges from explicit top/right/bottom/left face-center anchors
• reveal the measured result atomically, removing the architecture overlap baseline

Visual evidence

Before (gallery baseline):

After (this branch):

Verification

• full workspace test suite: 1,051 passed, 18 skipped
• gallery overlap suite: 28/32 clean, 4 known-bad, 0 regressed; architecture now PASS
• dedicated painted-DOM contract twice: no node/container overlap, no spillout, 32px member gap, standardized label padding, exact source/destination anchors, no edge-over-node crossings
• viewer production build

Stack

Depends on #305. Zones and swimlanes follow as separate stacked PRs.