feat(graph): deterministic force-directed layout solver (T-323)

Fruchterman-Reingold (clide-owned, no layout package): nodes repel, edges
attract, cooled over iterations into a readable layout. Deterministic — a
fixed circular seed, no RNG — so the graph view is stable across rebuilds
and the solver is unit-tested (6 cases). Flutter-free; the interactive graph
pane (render, pan/zoom, hover, filter, pql wiring) builds on this.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/src/graph/force_layout_test.dart

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+import 'dart:math' as math;
+
+import 'package:clide/src/graph/force_layout.dart';
+import 'package:test/test.dart';
+
+double _dist(GraphPoint a, GraphPoint b) => math.sqrt(math.pow(a.x - b.x, 2) + math.pow(a.y - b.y, 2));
+
+void main() {
+  test('an empty graph lays out to nothing', () {
+    expect(ForceLayout.compute(const [], const []), isEmpty);
+  });
+
+  test('a single node sits at the centre', () {
+    expect(ForceLayout.compute(['a'], const [], width: 800, height: 600), {'a': (x: 400.0, y: 300.0)});
+  });
+
+  test('every node settles inside the area', () {
+    final p = ForceLayout.compute(['a', 'b', 'c', 'd'], const [('a', 'b'), ('b', 'c'), ('c', 'd')], width: 800, height: 600);
+    expect(p, hasLength(4));
+    for (final pt in p.values) {
+      expect(pt.x, inInclusiveRange(0, 800));
+      expect(pt.y, inInclusiveRange(0, 600));
+    }
+  });
+
+  test('it is deterministic — same input, same layout', () {
+    final a = ForceLayout.compute(['a', 'b', 'c'], const [('a', 'b'), ('b', 'c')]);
+    final b = ForceLayout.compute(['a', 'b', 'c'], const [('a', 'b'), ('b', 'c')]);
+    expect(a, b);
+  });
+
+  test('two disconnected nodes repel farther apart than two connected ones', () {
+    final connected = ForceLayout.compute(['a', 'b'], const [('a', 'b')]);
+    final apart = ForceLayout.compute(['a', 'b'], const []);
+    expect(_dist(apart['a']!, apart['b']!), greaterThan(_dist(connected['a']!, connected['b']!)));
+  });
+
+  test('an edge to an unknown node is ignored, not a crash', () {
+    final p = ForceLayout.compute(['a', 'b'], const [('a', 'ghost')]);
+    expect(p.keys, containsAll(['a', 'b']));
+  });
+}