Cycle 4 of milestone #148 (Cost-model graph in R), decision E — the cost-graph
composite-builder, recorded across cycles 2-3 as the natural next.
Adds a `cost_graph(Vec<PrimitiveBuilder>) -> Composite` builder in aura-composites
that fans the 4 PM-geometry inputs to N cost nodes, surfaces each node's extra
inputs as `cost[k].<port>` roles (schema introspection past GEOMETRY_WIDTH),
sums them via CostSum, and exposes the 3-field aggregate. Replaces the CLI's
manual slot-indexed cost-wiring + the hardcoded MAX_RUN_COST_NODES=2 cap with
one principled composite of arbitrary arity. Re-exports GEOMETRY_WIDTH from
aura-std (first cross-crate consumer). Behaviour-preserving at the value level:
the composite inlines at bootstrap (C11) to the same flat computation, so the
two cycle-3 net_expectancy_r goldens, the C18 no-cost golden, and the full suite
stay green verbatim.
Routed spec-driven (specify direct-entry): settled in #148's scope + the mature
aura-composites idiom + the CostNode contract. All four forks (composite API
shape, extra-input namespacing, GEOMETRY_WIDTH visibility, CLI consumption) are
derived decisions logged on #148; none hangs on pure user preference.
Auto-signed under /boss on the Step-5 grounding-check PASS (every load-bearing
current-behaviour assumption tied to a named green test or current source; no
with-cost graph label/shape pin contradicts the C23-permitted label drift).
refs #148
