Add relations() virtual table tests and documentation

[khanaffan]

Summary

Add comprehensive tests and documentation for the new experimental relations() virtual table (ECVLib.Relations), which provides fast one-hop relationship traversal.

Depends on iTwin/imodel-native#1400 — the native vtab implementation.

Changes

Tests (core/backend/src/test/ecdb/RelationsVTab.test.ts)

• 16 active tests covering:
  • Link-table traversal (forward, backward, both, relationship class, unqualified syntax)
  • JOIN with entity class, subquery filtering
  • Multi-hop BFS, depth-limited BFS
  • Recursive CTE (full chain + depth limit)
  • Chained multi-hop via cross-join
  • Graph intersection and union
  • Edge cases (isolated node)
  • Real iModel (BIS relationships from root subject)
• 3 skipped (nav-prop and mixed — not yet supported by vtab)
• All queries use ECSQLOPTIONS ENABLE_EXPERIMENTAL_FEATURES

Documentation

• docs/learning/ECSqlReference/RelationsVTab.md — ECSQL reference: syntax, columns, examples (basic, directional, JOIN, cross-join, chained, recursive CTE, subquery), TypeScript usage, comparison table
• docs/learning/backend/RelationshipTraversal.md — Practical patterns: 6 patterns (neighbors, directional, JOINs, multi-hop CTE/cross-join/BFS, graph set ops, impact analysis)
• docs/changehistory/NextVersion.md — Changelog entry with experimental note and examples
• Navigation links added to index.md and leftNav.md

All docs include prominent experimental feature callouts noting the ECSQLOPTIONS ENABLE_EXPERIMENTAL_FEATURES requirement.

Validation

• Targeted verification: npx mocha --grep "relations" lib/cjs/test/ecdb/RelationsVTab.test.js — 16 passing, 3 pending
• Known baseline issues: None related to this PR