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-campaign/src/lib.rs

@@ -478,6 +478,7 @@ mod tests {
                 bindings: BTreeMap::new(),
             },
             risk: vec![],
+            cost: vec![],
             strategies: vec![StrategyEntry {
                 r#ref: DocRef::ContentId("0".repeat(64)),
                 axes: BTreeMap::from([(
@@ -905,6 +906,7 @@ mod wf_tests {
                 bindings: BTreeMap::new(),
             },
             risk: vec![],
+            cost: vec![],
             strategies: vec![StrategyEntry {
                 r#ref: DocRef::ContentId("c".repeat(64)),
                 axes: BTreeMap::from([(

crates/aura-campaign/tests/execute.rs

@@ -152,6 +152,7 @@ fn campaign(instruments: &[&str]) -> CampaignDoc {
             bindings: BTreeMap::new(),
         },
         risk: vec![],
+        cost: vec![],
         strategies: vec![StrategyEntry {
             r#ref: DocRef::ContentId(STRATEGY_ID.to_string()),
             axes: axes_2x2(),