audit(0082): cycle close — drift-clean; C10 cycle-0082 note + cost-flag validation symmetry

Cycle 0082 (cost-graph composition, cycle 2) closes drift-clean against C10. The
architect review confirms the cycle is fundamentally sound:

What holds: VolSlippageCost + CostSum are ordinary aura-std downstream nodes
(C9/C8), cost stays out of aura-engine; the vol proxy is a trailing realized range
(C2, no look-ahead); feed-forward, no equity feedback (C1); the co-temporality
contract (gate cost nodes on PM geometry only, 0-cost on a cold state input) is
consistent with C1/C2/C9 and the cycle-1 positional join, and charging 0 during
warm-up is honest (no estimate yet, not a fabricated guess). The no-cost C18 golden
is byte-identical.

Drift resolved at close:
- [medium -> fixed] docs/design/INDEX.md (C10): added the cycle-0082 realization
  note. The load-bearing co-temporality contract (generalizes to all future cost
  factors) lived only in code comments + spec 0082; the ledger is now its durable
  home once the spec is removed.
- [low -> fixed] crates/aura-cli/src/main.rs: --cost-per-trade now rejects negatives
  at parse, symmetric with --slip-vol-mult (it previously deferred to the node
  assert's panic). Run-path tests + the no-cost golden stay green.

Carry-on (documented deferrals, not this cycle's debt to fix):
- The 3-field cost triple {cost_in_r, cum_cost_in_r, open_cost_in_r} is restated
  by-convention across ConstantCost, VolSlippageCost, and CostSum (compiler-unlinked
  lockstep) — to be unified by the general CostNode trait, now justified by two
  concrete nodes (#148).
- SLIP_VOL_LENGTH=5 is sized to warm within the synthetic smoke fixture; documented
  in the const doc (on real M1 data any reasonable window warms).
- A future per-cycle-held accrual (carry/funding) accrues over holding duration,
  which will force a summarize_r fold change and stress the positional seam — flagged
  for that deferred cycle.

Regression gate green and unchanged: full workspace suite 0-fail, clippy --workspace
--all-targets -D warnings clean. No baseline moved -> no ratify.

Spec + plan ephemera removed (git rm docs/specs/0082, docs/plans/0082).

refs #148

crates/aura-cli/src/main.rs

@@ -3206,7 +3206,11 @@ fn parse_run_args(rest: &[&str]) -> Result<RunArgs, String> {
             }
             "--cost-per-trade" if cost.is_none() => {
                 let (value, t) = t.split_first().ok_or_else(usage)?;
-                cost = Some(value.parse().map_err(|_| usage())?);
+                let v: f64 = value.parse().map_err(|_| usage())?;
+                if v < 0.0 {
+                    return Err(usage());
+                }
+                cost = Some(v);
                 tail = t;
             }
             "--slip-vol-mult" if slip_vol_mult.is_none() => {