[codex] prepare sort_unstable feature release

[sstadick]

Summary

• Bump rust-lapper from 1.2.0 to 1.3.0 for the new sort_unstable feature.
• Add a README release-note entry for the feature flag with attribution to @jameslkingsley.
• Add construction benchmarks around Lapper::new and merge_overlaps to make the sort behavior easier to compare.
• Pin CI and local toolchain configuration to Rust 1.95.0, and update explicit iterator return lifetimes for compatibility.
• Gate the serde round-trip test behind with_serde so the default test suite passes without optional serde derives.