Add node groups and multi-view layout #12

Current State

Layout is non-semantic per-view metadata. The core persists node positions for one default view only; editors need node groups (titled frames around members) and more than one view of a document, persisted but never affecting evaluation.

Tasks:

• Groups with stable identities, titles and member elements, edited through reversible commands
• Multiple named views, each with its own positions and groups
• Persistence of groups and views inline, separately or omitted, with unknown view data preserved
• Removing a node updates group membership and restores it on undo

Acceptance Criteria:

• Groups and views never change validation or evaluation results
• Documents without groups or views keep their current serialized form
• Timing follows the editor persistence work in Add versioned graph persistence, migrations, and unknown-node preservation Guinevere#86

Related:

• Epic: MASS4.Graph core — headless model, identity, editing and persistence #1 (core epic), Add versioned JSON persistence, migrations and unknown-element preservation #9 (persistence)
• Add versioned graph persistence, migrations, and unknown-node preservation Guinevere#86