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/glossary.md

@@ -245,13 +245,15 @@ A node that converts a finer stream to a coarser bar stream, emitting a complete
 **Avoid:** risk section
 One entry of a campaign document's structural risk axis (`risk`): a
 serializable protective-stop regime (variants `vol{length,k}` — per-cycle —
-and `vol_tf{period_minutes,length,k}` — per completed time bucket) the matrix
-runs every cell under, so cells differ by execution discipline, never by
-signal. Absent or empty = one implicit default regime; the regime's stop
-defines the risk unit R — in `vol{length,k}` (stop = k·√EMA(Δ², length) over
-m1 cycles) `length` only smooths the vol estimator while `k` scales the stop
-distance, so the stop's timescale stays one cycle (`vol_tf` sets the stop's
-timescale via `period_minutes`).
+`vol_tf{period_minutes,length,k}` — per completed time bucket — and
+`fixed{distance}` — a constant price-unit distance, binding the shipped
+`FixedStop` composite) the matrix runs every cell under, so cells differ by
+execution discipline, never by signal. Absent or empty = one implicit default
+regime; the regime's stop defines the risk unit R — in `vol{length,k}` (stop =
+k·√EMA(Δ², length) over m1 cycles) `length` only smooths the vol estimator
+while `k` scales the stop distance, so the stop's timescale stays one cycle
+(`vol_tf` sets the stop's timescale via `period_minutes`; `fixed` has no
+timescale — the distance never adapts).
 
 ### run
 **Avoid:** —