docs: add Zenodo DOI to CITATION.cff and README badge

[simonsays1980]

Wires in the concept DOI (10.5281/zenodo.21984504) now that Zenodo has
archived the repo -- resolves to the latest release rather than pinning
to v0.1.3, matching Zenodo's own recommended practice. Adds the DOI
field to CITATION.cff and a badge to the README badge row.

arXiv identifiers/preferred-citation stay commented out in CITATION.cff;
no arXiv ID exists yet.

[simonsays1980]

✅ Safeguard benchmarks -- success

  speedups  : ['2.58x', '2.54x', '2.57x', '2.57x', '2.58x']
PASSED
  speedups  : ['2.50x', '2.51x', '2.50x', '2.53x', '2.48x']
PASSED
  speedups  : ['1.80x', '1.79x', '1.79x', '1.90x', '1.85x']
PASSED
  speedups  : ['2.66x', '2.66x', '2.68x', '2.70x', '2.87x']
PASSED
  speedups  : ['2.72x', '2.99x', '2.70x', '2.71x', '2.68x']
PASSED
  speedups  : ['2.31x', '2.31x', '2.32x', '2.34x', '2.36x']
PASSED
  speedups  : ['2.27x', '2.36x', '2.35x', '2.33x', '2.35x']
PASSED
  gae: PASSED
  vtrace: PASSED
  retrace: PASSED
  lambda_returns: PASSED
  discounted_returns: PASSED
  eligibility_traces: PASSED
  prefix_sum: PASSED
PASSED