Agents and users integrating the Bazel module routinely misread the
declared `eigen 3.4.0.bcr.3` dependency as a pin and conclude the
library only works with old Eigen. Under bzlmod it is a minimum:
version selection resolves to the highest Eigen requested anywhere in
the module graph, and every BCR eigen release from 3.4.0 through 5.x
shares compatibility_level 1.

Make this explicit in the README integration section and next to the
bazel_dep itself, and add a dedicated workflow that appends a
single_version_override for the newest BCR Eigen 5 release and runs
the full test suite, so the compatibility claim stays verified as new
Eigen versions land. The override is appended in CI rather than
committed because single_version_override is only honored in the root
module and would otherwise force the pin on consumers.