feat(graph): filter model — local-graph BFS, subgraph, GraphFilter (T-323)

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>

lib/src/graph/graph_filter.dart

@@ -0,0 +1,47 @@
+/// Client-side filtering of a loaded [VaultGraph] (T-323): narrow to the notes
+/// near the active one (depth-from-active), and/or by tag include/exclude.
+///
+/// Pure — the controller feeds in the already-queried tag map, so this runs
+/// under `dart test`. The file glob is NOT here: a different glob is a
+/// different file set, so it re-queries pql; these three refine what's already
+/// loaded.
+library;
+
+import 'package:clide/src/graph/vault_graph.dart';
+
+class GraphFilter {
+  const GraphFilter({this.depth, this.includeTags = const {}, this.excludeTags = const {}});
+
+  /// Hops from the active note to keep; null = the whole graph (no depth limit).
+  final int? depth;
+
+  /// Keep only notes tagged with at least one of these (empty = no filter).
+  final Set<String> includeTags;
+
+  /// Drop notes tagged with any of these.
+  final Set<String> excludeTags;
+
+  bool get isEmpty => depth == null && includeTags.isEmpty && excludeTags.isEmpty;
+
+  GraphFilter copyWith({int? depth, bool clearDepth = false, Set<String>? includeTags, Set<String>? excludeTags}) =>
+      GraphFilter(depth: clearDepth ? null : (depth ?? this.depth), includeTags: includeTags ?? this.includeTags, excludeTags: excludeTags ?? this.excludeTags);
+
+  /// Apply this filter to [full], using [tagsByPath] for the tag predicates and
+  /// [activePath] as the depth root. A depth filter with no active path (or one
+  /// that isn't a node) yields an empty graph — there's no local graph to show.
+  VaultGraph apply(VaultGraph full, {required Map<String, Set<String>> tagsByPath, String? activePath}) {
+    if (isEmpty) return full;
+    Set<String> tagsOf(String id) => tagsByPath[id] ?? const {};
+    var keep = {for (final n in full.nodes) n.id};
+    if (includeTags.isNotEmpty) {
+      keep = keep.where((id) => tagsOf(id).any(includeTags.contains)).toSet();
+    }
+    if (excludeTags.isNotEmpty) {
+      keep = keep.where((id) => !tagsOf(id).any(excludeTags.contains)).toSet();
+    }
+    if (depth != null) {
+      keep = activePath == null ? <String>{} : keep.intersection(full.nodesWithin(activePath, depth!));
+    }
+    return full.subgraph(keep);
+  }
+}