v0.7.0

Added

• Maximal clique enumeration: EnumerateMaximalCliques (Bron–Kerbosch with
  Tomita pivoting), a lazy iterative generator so the caller controls the
  cost of the exponential worst case by how far it enumerates. Direction is
  ignored, self-loops never count, and multigraph neighbors count once —
  the clustering-coefficient conventions. Deterministic order.

• Mermaid flowchart export: ToMermaid with MermaidExportOptions
  (vertex/edge label selectors and flow Direction). Nodes get synthetic
  insertion-order identifiers (v0, v1, …) with the display label attached
  at declaration, so arbitrary vertex strings are safe; labels use Mermaid
  entity escaping. Output is deterministic, like the DOT exporter.