designs: Balanced consolidation policy RFC

Add a new consolidationPolicy value, Balanced, that scores each
consolidation move by comparing savings and disruption as fractions of
NodePool totals. A consolidationThreshold parameter (default 2) controls
how aggressively the policy consolidates. A move passes when its
disruption fraction is at most k times its savings fraction.

The scoring formula is verified by exhaustive enumeration across three
instance families (c7i, m7i, r7i) at k=1 through k=5. k=2 is the
smallest value that makes within-family REPLACEs viable.

Author: jamesmt-aws