Move the canonical congressional-district geojson into this repo

[DTrim99]

Problem

The canonical congressional-district GeoJSON lives at
policyengine-app-v2/app/public/data/geojson/congressional_districts.geojson
— a frontend repo. Every PolicyEngine dashboard that renders a
choropleth (NC-Stein, WPTRA, NJ CTC/EITC, WV SB392, RCC, etc.) bundles
a copy. The per-district h5 files live here, but the polygons used to
build them live elsewhere.

This pretty much guarantees drift between:

• the polygons used to sample households into districts (the h5 build),
• the polygons rendered as the choropleth in each dashboard, and
• the polygons that hardcoded representative / region maps were
  authored against.

Ask

• Move (or duplicate + deprecate the app-v2 copy of)
  congressional_districts.geojson into policyengine-us-data
  alongside the h5 build script that consumes it.
• Publish the geojson on HF Hub as a sibling artifact to the h5
  set (e.g.,
  policyengine/policyengine-us-data/districts/geojson/{congress}.geojson).
• Update the app-v2 README to point at the new canonical home and
  flag the local copy as a cache.
• Long term: dashboards pull both the h5 and the matching-vintage
  geojson from a single @policyengine/district-boundaries package or
  HF URL.

Why now

Companion to #1143 (boundary-vintage metadata) and #1144 (119th → 120th
transition). Both are much harder to coordinate if the geojson is in a
different repo with a different release cadence than the data it
represents.

Related

• Document the boundary vintage of districts/*.h5 files #1143 — document boundary vintage on the existing h5 files
• Plan the 119th → 120th boundary transition (8 states) #1144 — 119th → 120th boundary transition (8 states)
• Audit 118th → 119th boundary catch-up (GA, NC, NY, AL, LA) #1146 — 118th → 119th boundary catch-up audit