Fix #1115: honor KthLargestMTuple threshold

[isPANN]

Summary

• replace aggregate tuple counting with the documented K-threshold decision result
• enumerate the Cartesian product iteratively with overflow-safe bound comparison and early exit at K
• update canonical examples, paper documentation, unit coverage, and CLI regression coverage

Verification

• make check
• make paper
• focused KthLargestMTuple unit tests
• focused CLI k=14/k=15 regression
• cargo clippy --all-targets --features ilp-highs -- -D warnings

Stack

• Base: Symbolic growth, exact Pareto path search, and deterministic solver backends #1083 (1075-growth-domain)

Fixes #1115