audit(0081): cycle close — drift-clean (code); ledger C10 realization note + stale-broker doc sync (refs #148)

Cycle 0081 (cost-model graph, cycle 1) closes drift-clean against C10: the
architect review confirmed ConstantCost + the net-R seam realize the contract
at its floor (C9 cost node; net_r_equity a C8/C18 sink; R-pure; one home for
cost, subsuming the retired scalar round_trip_cost; pure feed-forward C1), and
the no-cost baseline is byte-identical (C18 golden held). Regression gate green
and unchanged: full workspace suite 0-fail, clippy --workspace --all-targets
-D warnings clean. No baseline moved -> no ratify.

Ledger sync (docs/design/INDEX.md): added the C10 cycle-0081 realization note
(ConstantCost node + net_r_equity seam shipped; scalar round_trip_cost retired;
run-path-scoped; later-cycle deferrals named).

Drift swept (cycle-adjacent + cheap one-liners):
- crates/aura-analysis/src/lib.rs net_expectancy_r doc: the removed in-fold
  mechanic (mean(R - round_trip_cost/latched_dist)) -> the cost-stream fold.
- crates/aura-std/src/lib.rs module doc: "realistic broker profiles" (retired
  by #116) -> the legacy SimBroker pip yardstick + cost-model nodes.

Backlogged (pre-existing, broader, predates this cycle): the
docs/project-layout.md backtest worked example is stale on #117 (exposure->bias)
and #116 (realistic broker); a proper rewrite is filed as #151, not folded here.

Architect-accepted: the #[allow(dead_code)] on r_col::ENTRY_PRICE/STOP_PRICE
(their production reader legitimately removed when summarize_r stopped recovering
latched_dist; still the layout contract + exercised by test fixtures).

Spec + plan ephemera removed (git rm docs/specs/0081, docs/plans/0081).

refs #148

crates/aura-std/src/lib.rs

@@ -2,8 +2,8 @@
 //!
 //! The universal, batteries-included building blocks every project gets for
 //! free: common indicators (SMA, ATR, RSI, …), resamplers, the `SessionNode`,
-//! standard combinators, and broker nodes (the sim-optimal broker + realistic
-//! broker profiles). Depends only on `aura-core` (the shared contract).
+//! standard combinators, the legacy `SimBroker` pip yardstick, and cost-model
+//! nodes (`ConstantCost`, …, in R — C10). Depends only on `aura-core` (the shared contract).
 //!
 //! This is the top tier of the three-tier node-reuse model (contract C16):
 //!