Lift the duplicated cost-node skeleton — shared verbatim by ConstantCost and
VolSlippageCost and locked by convention against CostSum — into one abstraction:
- A new aura-std/src/cost.rs owns the cost-record contract (COST_WIDTH,
COST_FIELD_NAMES, GEOMETRY_WIDTH), the CostNode factor trait (one hook:
cost_numerator, in price units), the generic CostRunner<F> adapter that holds
the shared state (cum, out) and the co-temporality skeleton (geometry gating,
numerator/latched R-normalization, closed/open charge, 3-field emit), and a
cost_node_builder schema assembler.
- ConstantCost and VolSlippageCost become thin CostNode factors; their new()
returns CostRunner<Self>, so every existing call site and unit test binds
transparently and stays green verbatim.
- CostSum and main.rs drop their local triple consts for the shared
COST_FIELD_NAMES — the cycle-2 audit-flagged by-convention 3-field lockstep is
now a structural single-source contract (the four copies of the triple collapse
to one). main.rs also reads COST_WIDTH in place of the literal slot stride.
Behaviour-preserving: the builders emit unchanged schemas (same port names), so
the wiring, the net_r_equity seam, and summarize_r are untouched; the
numerator/latched token form is preserved verbatim (IEEE-754 byte-identity). The
existing suite is the regression net — all green: the per-node unit tests pass
verbatim (0.5/1.5, 0.375/1.375), the composition E2E exact, the C18 no-cost
golden byte-identical. Two new CLI characterization goldens pin the exact
net_expectancy_r of the flat and composed cost paths (the prior tests only
asserted net < gross, which a numerator drift would pass silently). New cost.rs
runner/contract tests + the HalfSpreadCost author doctest cover the new surface.
Deviation from the plan: CostNode::name() was dropped (the plan over-specified
it) — it is dead surface, nothing consumes it (CostRunner forwards label();
cost_node_builder takes the name as an explicit arg). Removed from the trait,
both impls, the doctest, and the test stubs; build + suite + clippy green.
Verified: cargo build --workspace --all-targets clean; cargo test --workspace
0 failures; cargo clippy --workspace --all-targets -- -D warnings clean; doctest
green.
refs #148
Brummel
6b53c239dd