Cycle 1 of the "Cost-model graph (in R)" milestone (#148): a single
ConstantCost node (flat round-trip cost per trade) plus the net-R seam —
an in-graph cost_in_r stream, a net_r_equity sink (sibling of r_equity),
and summarize_r folding that one stream into net_expectancy_r (replacing
the scalar round_trip_cost). A run with no cost stays a byte-identical
gross-R baseline. Defers the general CostNode trait, the cost-graph
composite-builder, data-grounded factors, and the conviction axis.
Auto-signed under /boss on the grounding-check PASS: all load-bearing
existing-behaviour assumptions are ratified by green tests. Fork
decisions recorded on #148.
refs #148
Brummel
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