docs(ledger,research): note the metric-vocabulary drift guard (#190 close)

The #190 cycle-close audit found two now-stale claims that predate this
cycle's guard test: the ledger's known-debt note listed the metric-roster
triplication as tracked by #190 (the issue that just added the guard, closing
on push), and metric_vocabulary()'s doc comment told a hand-editor to keep the
list in sync by hand without mentioning that a red test now catches a desync.

Reconcile both to the honest state: the triplication remains (still a
hand-list) but drift from the shipped aura-analysis types is now caught by the
cross-crate guard `aura-campaign/tests/metric_vocabulary_e2e.rs`; the
single-source removal that would delete the hand-list is tracked under #147
(where the metric vocabulary's home is decided).

refs #190, #147

docs/design/INDEX.md

@@ -1473,7 +1473,9 @@ are validated against), and a blueprint-file mode on `--content-id`. The
 both `RollMode`s; the shipped `folds` slot mapped to nothing the machinery accepts), with
 a new `ZeroWalkForwardLength` intrinsic fault; wf-bearing process docs get new content ids
 by design (the 0106 fieldtest corpus stays as the historical record). Known debt:
-metric-roster triplication (#190), deflation-constant duplication (#199).
+metric-roster triplication (still hand-maintained, but drift from the shipped
+`aura-analysis` types is now test-caught by a cross-crate guard, #190;
+single-source removal waits on #147), deflation-constant duplication (#199).
 **Realization (cycle 0108, #200 — the annotator stages execute).** The v2 executable
 shape is `std::sweep [std::gate]* [std::walk_forward]? [std::monte_carlo]?
 [std::generalize]?` — an ordered optional annotator suffix, each at most once,