feat(aura-research, aura-runner): RiskRegime::Fixed — the fixed stop is campaign-reachable

Harvest sweep, batch 3 of 6 (second half).

The third externally-tagged variant ('{"fixed":{"distance":..}}',
additive — stored Vol/VolTf documents keep their bytes): doc-tier
validation mirrors FixedStop::new's distance>0 assert (NaN refused)
so the assert is unreachable from a document; stop_rule_for_regime
binds the shipped FixedStop composite; run_blueprint_member stamps
stop_distance beside the vol knobs' precedent and
stop_rule_from_params re-derives Fixed from it (stamps are mutually
exclusive; the pre-#233 no-knob fallback is unchanged). BadRegime
prose widened to name the distance rule. C10's risk-axis paragraph
updates from 'additive when needed' to shipped; glossary gains the
fixed variant.

closes #338

crates/aura-cli/src/research_docs.rs

@@ -161,7 +161,7 @@ pub(crate) fn doc_fault_prose(f: &DocFault) -> String {
             format!("data.windows[{index}]: from_ms must be earlier than to_ms")
         }
         DocFault::BadRegime { index } => {
-            format!("risk[{index}]: stop length must be >= 1 and k must be > 0")
+            format!("risk[{index}]: stop length must be >= 1, k must be > 0, and a fixed-stop distance must be > 0")
         }
         DocFault::BadCost { index } => {
             format!("cost[{index}]: the component's price-unit knob must be finite and >= 0")