feat: add store.subgraph() for typed neighborhood extraction

[pdlug]

Summary

• Adds store.subgraph(rootId, options) for BFS subgraph extraction via recursive CTE
• Supports multi-edge-kind traversal, bidirectional edges, includeKinds filtering, excludeRoot, cycle detection (prevent/allow), and soft-delete exclusion
• Returns fully typed discriminated union results (SubsetNode<G, NK>[], SubsetEdge<G, EK>[]) with compile-time narrowing via includeKinds
• Dialect-aware SQL: SQLite uses CROSS JOIN + INSTR path for worktable ordering; PostgreSQL uses standard JOIN ON + array-based cycle detection

What shipped

Type utilities (zero runtime cost):

• AnyNode<G> — discriminated union of all runtime node instances
• AnyEdge<G> — discriminated union of all runtime edge instances
• SubsetNode<G, K> — narrowed to selected node kinds
• SubsetEdge<G, K> — narrowed to selected edge kinds

store.subgraph(rootId, options):

• BFS traversal via recursive CTE following a set of edge kinds
• maxDepth (default 10, capped at MAX_RECURSIVE_DEPTH)
• includeKinds — filter result nodes while still traversing through excluded kinds
• excludeRoot — omit the root node from results
• direction: "out" | "both" — unidirectional or bidirectional traversal
• cyclePolicy: "prevent" | "allow" — dialect-aware cycle detection
• Handles SQLite worktable join ordering (CROSS JOIN pattern)
• 2 parallel SQL queries (nodes + edges), each self-contained with the CTE
• Edges filtered to those where both endpoints are in the final node set

Closes #31