Link to Axis2/C parity implementation from Java financial sample

Adds @see javadoc links to the C equivalent header and implementation
(financial_benchmark_service.h/.c in axis-axis2-c-core) from the Java
FinancialBenchmarkService class-level javadoc. Both implementations
maintain identical financial calculations, API contracts, and edge-case
handling — the cross-link makes the parity relationship discoverable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>