Prepare 3.0.0 release #68

Let's prepare 3.0.0 for the new matrix and vector features from #44 and the math fixes from #50.

• Bump mix.exs to 3.0.0 and update the README installation example.
• Add release notes for the new APIs and the behavior changes callers need to account for when upgrading.
• Correct the August 2024 changelog heading from 2.5.0 to 2.6.0.
• Check formatting, compilation, the full coverage suite, and package metadata.

Prepared in #69. Local validation passes with 597 tests, 7 doctests, and 100% library line coverage. The package builds as 3.0.0; tagging and publication follow after merge.