docs: add FAPP research paper LaTeX sources

Add paper directory with LaTeX source, bibliography, and Makefile
for the FAPP (Federated Application Protocol) research paper.
Build artifacts are gitignored.

.gitignore

@@ -26,3 +26,11 @@ docs/internal/
 # AI development workflow files
 master-prompt.md
 scripts/ai_team.py
+
+# LaTeX build artifacts
+paper/*.aux
+paper/*.bbl
+paper/*.blg
+paper/*.log
+paper/*.out
+paper/*.pdf