Improve ranked local placer sampling

[rollrat]

Summary

• add ranked future-join/fanout-aware placement sampling
• preserve geometry-diverse beam candidates during ranked sampling
• reuse shared bounding-box estimates for placement diversity
• tune the full-adder generated fixture test to use ranked sampling with route depth 6

Verification

• cargo test --release transform::place_and_route::estimate -- --nocapture
• cargo test --release transform::place_and_route::local_placer::unit_tests -- --test-threads=1
• cargo test --release test_generate_component_full_adder -- --ignored --nocapture