Add CPU FGMRES support and correct BlockSchur factorization

[chvillanuevap]

Summary

This PR introduces two independent improvements to Smith's linear solver infrastructure:

1. CPU FGMRES and configurable restart length

   • Exposes native MFEM FGMRES through Smith's solver factory.
   • Adds configurable Krylov restart dimensions for GMRES and FGMRES.
   • Includes factory, configuration, and solver tests.

2. BlockSchur factorization correction

   • Eliminates redundant application of the first-block inverse in lower/full factorization.
   • Preserves the previously computed velocity inverse result.
   • Adds tests for block algebra, in-place application, and inverse application counts.

Validation

• 395 tests passed across the three affected test executables.
• Both commits were checked for integration conflicts with Roy's custom-Schur branches. No production-code conflicts were identified; combined compilation remains pending.

Scope

This PR does not introduce new custom-Schur APIs.