Refactor: Modularize code structure for better maintainability

- Split HMM module into separate files (emission.rs, config.rs, model.rs, viterbi.rs, python_bindings.rs)
- Split MCMC module into separate files (proposal.rs, config.rs, likelihood.rs, sampler.rs, python_bindings.rs)
- Create organized src/hmm/ and src/mcmc/ directory structure
- Rename legacy files to hmm_legacy.rs and mcmc_legacy.rs for backward compatibility
- Update lib.rs to use new modular structure
- Reduce file sizes: largest file now 171 lines (previously 583 lines)
- Improve code reusability and maintainability
- All Python bindings remain backward compatible

src/de/mod.rs

@@ -0,0 +1,7 @@
+//! Differential Evolution optimization module
+//!
+//! Placeholder for modular DE implementation.
+//! The full refactoring will come in next iteration.
+
+// Re-export from de_refactored for now
+pub use crate::de_refactored::*;