Dictionary schema extensions: unit and allowed_values (blocked by washr spike)

[larnsce]

Milestone: v1.3.0 (attach once the milestone exists). Blocked by #37; do not start unless the washr round-trip spike passed. Part of the v1.1 roadmap: docs/roadmap-v1.1.md.

Tasks

• Optional unit and allowed_values columns in data-raw/dictionary.csv (units for numeric variables; factor level sets, in order for ordinals, since CSV exports cannot carry levels).
• Backfill protocol, from premortem finding F6 (an agent that infers units and allowed values from the data and then tests against its own inference is a self-certifying circle): the agent may scaffold the new columns as TODO cells; the values are human-authored; inferred values never merge unedited.
• Advisory checklist item referencing the extended schema; reconciliation entries; fixture and gate updates.

Kill criterion

If #37 failed or is unresolved, close this as not planned rather than shipping a schema the toolchain destroys.