Clarify gainlineup agent usage

[iancleary]

Summary

• add an Agent Usage section for choosing gainlineup for ordered RF block chains
• mirror the crate-selection guidance in README.md and crate-level docs for dependency consumers
• clarify handoffs to touchstone, linkbudget, and rfconversions
• document stage modeling conventions for gain, losses, output-referred P1dB/IP3, and input noise temperature

Verification

• cargo fmt -- --check
• cargo test