Nested subgraph support: Phase 1 parser and graph model

[dnvt]

Summary

Start Phase 1 of nested subgraph support by adding parent/child subgraph structure in the parser and graph model.

Scope

• add parent/child metadata to graph subgraphs
• preserve nested subgraph structure during parse
• keep node membership associated with the innermost subgraph
• add parser/model tests for nested hierarchy

Context

Tracked by planning/NESTED_SUBGRAPH_SUPPORT.md and analysis/2026-04-08-nested-subgraph-containment.md.

Out of Scope

• hierarchical layout/envelope containment
• nested portal routing
• nested border rendering polish