PAPER: Add Russell R P Senthamarai as co-author

Russell missed the invitation window on issue #94 due to GitHub access
issues and confirmed authorship afterward. He is a code contributor
(dot product for Dual numbers, with tests). Inserted in alphabetical
order by family name (between Sen and Sörngård) in paper.md and
CITATION.cff.

Author: Axect

CITATION.cff

@@ -33,6 +33,10 @@ authors:
     family-names: Sen
     affiliation: Independent Researcher
     orcid: 'https://orcid.org/0009-0008-0337-9964'
+  - given-names: Russell R P
+    family-names: Senthamarai
+    affiliation: Independent Researcher
+    orcid: 'https://orcid.org/0000-0002-8061-5480'
   - given-names: Johanna
     family-names: Sörngård
     affiliation: Independent Researcher

paper/paper.md

@@ -27,6 +27,9 @@ authors:
   - name: Soumya Sen
     orcid: 0009-0008-0337-9964
     affiliation: 4
+  - name: Russell R P Senthamarai
+    orcid: 0000-0002-8061-5480
+    affiliation: 4
   - name: Johanna Sörngård
     orcid: 0000-0002-8660-9989
     affiliation: 4