v2.7.1: Discrete distributions gold standard upgrade & HTK continuous benchmarking

Major Features:
• Elevated all 4 discrete distributions (Discrete, Binomial, Negative Binomial, Poisson) to gold standard
• Comprehensive exception handling in stream input operators
• Robust boundary value management and input validation
• Consistent variable naming conventions across discrete distributions

Benchmarking Enhancements:
• Clean separation of discrete vs continuous HTK benchmarks
• Comprehensive 1D Gaussian continuous distribution benchmarking
• Extended performance scaling analysis (100 to 1,000,000 observations)
• Performance crossover analysis: libhmm excels <5k obs, HTK dominates >10k obs

Updated Performance Rankings (latest benchmarks):
• GHMM: 25,164.9 obs/ms (24.25x faster than libhmm)
• HMMLib: 18,889.2 obs/ms (17.83x faster than libhmm)
• StochHMM: 2,075.7 obs/ms (2.10x faster than libhmm)
• libhmm: 1,037.6 obs/ms (baseline)
• HTK: Variable scaling (177x faster for very long sequences)

Quality Improvements:
• 100% test pass rate maintained
• Enhanced numerical robustness and error handling
• No breaking changes - full backward compatibility
• Comprehensive documentation updates

Files Modified:
• All 4 discrete distribution headers and implementations
• HTK benchmark separation and continuous support
• Updated performance rankings and scaling analysis
• Comprehensive benchmarking documentation updates"