fix(visualization): make auto mode consider edge count, fall back to file aggregation (#609)

Auto mode switched away from the full D3 force layout only when node
count exceeded 3000; edge count was ignored, so a 2792-node/17488-edge
graph stayed in full mode and the force layout stalled the browser
(O(E) link force per tick plus one SVG element per edge).

- Add DEFAULT_MAX_FULL_EDGES = 3 * DEFAULT_MAX_FULL_NODES (9000),
  derived from the ~3 edges/node budget the node cap already tolerated
- Auto mode now switches when rendered nodes > max_full_nodes OR
  rendered edges > max_full_edges
- When switching without community data, fall back to file aggregation
  instead of collapsing everything into one Uncategorized super-node
- Regression tests cover the exact reported 2792/17488 boundary

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FmPUotoqsNWWt3b41FMjwv