[Enhancement] Zone support — per-node zone field with visual card accent

[rseldner]

Summary

Add a zone field to TREE nodes that applies a subtle visual accent (left-border color or background tint) to group nodes by logical layer, without affecting layout.

Motivation

Troubleshooting trees that span multiple system layers (e.g., Client Browser → Platform Services → System Clusters in ECE) have no way to visually indicate which layer a step belongs to. Node type already conveys what kind of outcome a node is — zone would convey where in the system it operates.

Design

• New optional field on any TREE node: zone: "client-browser"
• Zones defined in CONFIG (name + color), e.g. CONFIG.zones
• Engine reads zone at rebuildTree() time and applies a colored visual accent to the card — distinct from and composable with node type styling
• No layout changes; zone membership doesn't need to be spatially contiguous
• Zones appear in the legend alongside node types, or as a separate legend section

Non-goals

• No swimlane-style background panels or bounding boxes
• No constraint on which nodes can share a zone
• No change to agent traversal behavior

Open questions

• Legend treatment: same legend row as node types, or a second legend section?
• Accent style: left-border stripe (already used by type), background tint, or a badge/pill?
• Playground: zone picker in the node editor panel (follow-up, not blocking)