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

docs/design/contracts/c10-bias-r-cost.md

@@ -284,10 +284,11 @@ with like; costed families reproduce bit-identically (incl. `Carry`).
 curves; a cost-less doc requesting it keeps a remedy-naming skip notice. There are
 no `--cost-*` run-path flags (removed #221/#234); cost travels in the document.
 
-**Risk regime as a structural campaign axis.** The `StopRule{Fixed, Vol}` axis is
-realized at the campaign-document level as `CampaignDoc.risk: [RiskRegime]`
-(`aura-research`, variants `Vol{length, k}` and `VolTf{period_minutes, length, k}`
-(#262), the fixed-stop rule additive when needed) — a kept-separate matrix axis,
+**Risk regime as a structural campaign axis.** The `StopRule{Fixed, Vol, VolTf}`
+axis is realized at the campaign-document level as `CampaignDoc.risk:
+[RiskRegime]` (`aura-research`, variants `Vol{length, k}`,
+`VolTf{period_minutes, length, k}` (#262), and `Fixed{distance}` (#338, binding
+the shipped `FixedStop` composite)) — a kept-separate matrix axis,
 peer of instruments and windows. The executor keys the nominee map by `(strategy,
 window, regime)`, so `generalize` aggregates *within* a regime, never across.
 Regimes are **compared** at presentation, never argmax-**selected** across (a