feat(research,cli): the CostSpec campaign vocabulary (#234 task 2)

CampaignDoc gains an additive cost: Vec<CostSpec> block — a closed,
externally tagged vocabulary over the three shipped cost nodes
(constant/vol_slippage/carry, field names = the builders' ParamSpec
names), mirroring the risk block: serde default + skip-if-empty (cost-
less documents keep their content ids, pinned), DocFault::BadCost
validation (finite/bounds per component), std::cost construct-
vocabulary block + SlotKind::Costs + open-slot mirror, prose arm, and
all CampaignDoc struct literals across crates carry cost: vec![]
(compile-driven).

Verified: aura-research suite green, campaign validate e2e green, full
workspace suite green, clippy -D warnings clean; in-loop spec review
compliant and quality review approved (real diff fingerprint — the
worktree-anchored review prompt works).

refs #234

crates/aura-cli/src/research_docs.rs

@@ -144,6 +144,9 @@ pub(crate) fn doc_fault_prose(f: &DocFault) -> String {
         DocFault::BadRegime { index } => {
             format!("risk[{index}]: stop length must be >= 1 and k must be > 0")
         }
+        DocFault::BadCost { index } => {
+            format!("cost[{index}]: the component's price-unit knob must be finite and >= 0")
+        }
         DocFault::NoStrategy => "strategies is empty".into(),
         DocFault::EmptyAxes { strategy } => format!("strategies[{strategy}]: axes is empty"),
         DocFault::EmptyAxis { strategy, axis } => {