feat(0085): per-cycle-held accrual — CarryCost + ChargeMode (approach B)

Cycle 5 of milestone #148 (cost-model graph in R): the per-cycle-held accrual
mechanism — C10's "per-cycle-held factors accrue over the hold". A carry/funding cost
is charged for every cycle a position is held, so the net_r_equity equity curve bleeds
continuously over the hold (approach B, "ja, mach B") rather than stepping at close.

Mechanism (logged on #148): ChargeMode { AtClose, PerHeldCycle } is a property of the
cost factor, read by the one shared CostRunner (no second runner type). The AtClose arm
is the pre-cycle-5 eval tokens VERBATIM (IEEE-754 byte-identity). The PerHeldCycle arm
accrues `per` into `acc` each held cycle, dumps the accrued total into `cum` at close
(resetting acc), and marks the open position via a growing open_cost_in_r. The bleed
lives in open_cost_in_r, which the net_r_equity tap already subtracts — so summarize_r
and the CLI net_eq wiring are UNCHANGED, and the cycle-0083/0084 net_expectancy_r
goldens (-614.3134020253314 flat, -615.0304388396047 composed) + the C18 no-cost golden
stay byte-identical.

CarryCost is a ConstantCost twin differing only in charge_mode() -> PerHeldCycle (a
labelled stress parameter, the flat base of the accrual family). Wired through the
existing cost_graph/CostSum aggregation; a per-trade ConstantCost and a per-held-cycle
CarryCost compose in one run (the costs sum per-field — the composed golden is exactly
the sum of the two single-cost charges). New run-path --carry-per-cycle flag
(sweep/walkforward/mc pass None, as the existing cost flags do).

Tests: 2 RED-first unit B-proofs (open_cost grows over the hold, dumps the total at
close, acc resets between trades — the discriminator a scalar net_expectancy_r cannot
see); the CarryCost twin's 5 unit tests; 2 captured net_expectancy_r goldens
(-768.2095026600887 carry, -1383.7939051991182 composed); a net_r_equity-bleeds-over-
the-hold integration test (the terminal open hold's r_equity-net_r_equity gap strictly
grows); plus negative-rate-refused-exit-2, monotone-in-rate, and zero-rate-exactly-free
guards.

Verified: cargo test --workspace (0 failures), cargo build --workspace, cargo clippy
--workspace --all-targets -- -D warnings clean. summarize_r / aura-analysis untouched.
Deferred to later #148 cycles (logged there): notional-based carry, the calendar-aware
overnight swap, sweep-path cost.

refs #148

crates/aura-std/src/lib.rs

@@ -20,6 +20,7 @@
 mod add;
 mod and;
 mod bias;
+mod carry_cost;
 mod constant_cost;
 mod cost;
 mod cost_sum;
@@ -49,8 +50,12 @@ mod vol_slippage_cost;
 pub use add::Add;
 pub use and::And;
 pub use bias::Bias;
+pub use carry_cost::CarryCost;
 pub use constant_cost::ConstantCost;
-pub use cost::{cost_node_builder, CostNode, CostRunner, COST_FIELD_NAMES, COST_WIDTH, GEOMETRY_WIDTH};
+pub use cost::{
+    cost_node_builder, ChargeMode, CostNode, CostRunner, COST_FIELD_NAMES, COST_WIDTH,
+    GEOMETRY_WIDTH,
+};
 pub use cost_sum::CostSum;
 pub use delay::Delay;
 pub use ema::Ema;