feat: serialize MCMC diagnostics

[kholdrex]

Summary

Adds optional serde serialization support for bayes-rs MCMC diagnostics and multi-chain outputs. This makes convergence summaries, trace plots, and multi-chain run results easier to export to JSON or other serde-backed formats without changing the default dependency surface.

Changes

• Adds an optional serde feature that enables serialization derives for diagnostics and multi-chain output types.
• Adds serde-covered tests for diagnostics, trace plots, summaries, and multi-chain output.
• Adds a serde_diagnostics example and README guidance for exporting diagnostic summaries.

Test plan

• Rust formatting passed.
• Full all-features test suite passed.
• Clippy passed with warnings denied.
• Documentation build passed with warnings denied.
• Benchmark targets compiled successfully.