Add citation for ECT completeness claim

[raphaelreinauer]

Per the JOSS review checklist:

Is the list of references complete, and is everything cited appropriately that should be cited?

The Summary states the ECT "provid[es] a complete topological summary" without a direct citation.
This refers to the injectivity result. Consider adding a citation here, e.g., Turner2014 or Munch2025.

Part of this JOSS review.