Rename gradient_barriers_minimal -> gradient_barriers_break (no longer minimal-reduced)

[NewGraphEnvironment]

Since #223, gradient_barriers_minimal is fed the full per-model raw barrier
positions (every gradient/falls barrier splits the network), NOT the
fresh::frs_barriers_minimal() downstream-most reduction. The name "minimal" is now
misleading. Rename to gradient_barriers_break (or similar) across:

• R/lnk_pipeline_prepare.R (the build + union)
• R/lnk_pipeline_break.R (the gradient_minimal break source + roxygen table)
• docs / tests referencing the name

Deferred from #223 to keep that PR's diff focused (agreed with user). Relates to #223.