Adds the pure filtering primitives the graph pane composes: VaultGraph
nodesWithin (depth-bounded BFS over undirected edges = the local graph
around a note) and subgraph (retain a node set + the edges between them),
plus GraphFilter, which combines depth-from-active with tag include/exclude
over a caller-supplied tag map. Flutter-free; runs under dart test.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>