Fixes #207 | faithful Eades (1984) Spring Embedder implementation

The previous implementation deviated from Eades (1984) in
ways that caused poor layouts and unnecessary slowness.

Visual quality fixes:
- Remove f_rep=0 cutoff for dist > 2*optimalDistance — this was the
  primary cause of node clustering; all pairs now repel unconditionally
- Fix naturalLength (c2): was 2.5*computeOptimalDistance (~297px for
  N=100), now canvasMinDimension/sqrt(V) (~80px) — the correct
  pixel-space equivalent of Eades' unit-square formulation; the old
  value made log(d/c2)≈0 for most pairs, nearly disabling attraction
- Revert c_rep (c3) to 1.0 per Eades (1984); revert c4_init to 0.1
- Add mild linear cooling schedule (c4: 0.10→0.01) to help settling
- Add displacement cap (canvasMinDimension*0.05) to prevent
  early-iteration force explosions from near-coincident nodes
- Add early-exit convergence when maxDisp < epsilon (2.0px)
- Reduce maxIterations from 500 to 300 in slotLayoutSpringEmbedder()
- Warn user when N>30 (Eades' own stated limitation of the algorithm)

Performance fixes:
- Pre-cache adjacency into QSet<QPair<int,int>> before iteration loop;
  replaces O(log N) edgeExists() calls in the O(N²) inner loop
- Inline Eades force math directly; eliminates per-call QString dispatch
- Cache vertex numbers to local const int before inner loop
- Emit setNodePos in a single bulk pass after the iteration loop

Author: oxy86

src/graph.h

@@ -1079,8 +1079,6 @@ public slots:
 
     void layoutRandom();
 
-    void layoutRandomInMemory();
-
     void layoutRadialRandom(const bool &guides = true);
 
     void layoutCircular(const double &x0,
@@ -1122,7 +1120,7 @@ public slots:
                                     const qreal &dist,
                                     const qreal &optimalDistance);
 
-    void layoutForceDirected_Eades_moveNodes(const qreal &c4);
+    qreal layoutForceDirected_Eades_moveNodes(const qreal &c4);
 
     void layoutForceDirected_FR_moveNodes(const qreal &temperature);
 
@@ -1243,6 +1241,8 @@ public slots:
                         H_StrToInt &discreteClasses,
                         int &classes, int name);
 
+    void layoutRandomInMemory();
+    
     VList m_graph; // List of pointers to the vertices. Each vertex stores all info: links, colors, etc
 
     Parser *file_parser; // Our file loader threaded class.