feat(wordram): prepare and encode all shared matrix bands - #1536

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

`Cslib.Algorithms.WordRAM.MatrixMultiplication.Schonhage.BandPreparation.headerRegisters` and `Cslib.Algorithms.WordRAM.MatrixMultiplication.Schonhage.BandPreparation.basicRegisters` (private) (Complexity/Algorithms/WordRAM/MatrixMultiplication/Schonhage/BandHeader.lean:73) share a 267-node subterm, 54% of the smaller body, built from `Insert.insert`, `Finset`, `Cslib.Algorithms.WordRAM.MatrixMultiplication.Schonhage.BandPreparation.two`, `Cslib.Algorithms.WordRAM.MatrixMultiplication.Schonhage.BandPreparation.three`, `Cslib.Algorithms.WordRAM.MatrixMultiplication.Schonhage.BandPreparation.four`, `Cslib.Algorithms.WordRAM.MatrixMultiplication.Schonhage.BandPreparation.seven`. Advisory only; see the job summary.