Prod-enablement checklist for TAXONOMY_AUTHORING_ENABLED (disclosure/text-block authoring)

[jfrench9]

Summary

Tracking checklist for turning on TAXONOMY_AUTHORING_ENABLED (the
disclosure-note + text-block authoring surface) in prod/staging. It ships
dark in v1.6.6 (default false in prod/staging), which is intentional — this
issue is the gate for flipping it on.

The dark-gating is load-bearing: FactLite.value only goes null for non-numeric
(text-block) facts, which can only exist once concrete text-block elements are
authored — which requires this flag. So none of the items below are live at the
v1.6.6 tag; they become required the day the flag flips.

Before flipping the flag in prod

• Nullable value client/frontend compat. FactLite.value is now
  float | None (backward-incompatible OpenAPI change; non-numeric facts
  return null). Confirm @robosystems/report-components, robosystems-app,
  and roboledger-app tolerate a null value; regen/bump the TS + Python
  SDKs if they typed it as a required number. (robosystems-client dev dep is
  already at 0.5.3.)
• Metrics FactSet uniqueness landed — see the partial-unique-index issue.
• bind-text-block hardening landed (deterministic element resolution +
  structure-membership check; library-immutable item_type guard) — see the
  hardening issue.
• LIFO-only period reopen confirmed. reopen_period validates only
  status == "closed" with no LIFO guard; reopening an interior (non-most-
  recent) closed period flips FiscalPeriod.status → "closing" but leaves the
  graph watermark and schedule fact_scope untouched, so status and
  schedule-fact scope disagree. Either enforce boundary-only reopen in the
  reopen API, or document interior reopen as unsupported.

Notes

• Set the flag via SSM (just ssm-set {env} features/TAXONOMY_AUTHORING_ENABLED true) — takes effect at runtime, no redeploy.

Found during the v1.6.6 pre-release review.