Docs: sync pass — match the validator + current architecture

The docs drifted as the project moved fast. Fixed:
- Counts to the validator (canonical): 105,743 records, Jurisdiction 29,905
  (README had 105,746/29,908 — the exact-GEOID backfill dropped 3 city nodes
  and only the commit message was updated, not the docs).
- sources.md: officeholders-v2 → v3 (13,329); added the inputs that landed
  since — municipal GEOID backfill, place/district crosswalks, the complete
  U.S. Code corpus, the executive offices, and the authority edges.
- board.md: it described income/population/poverty only; now names the
  partisan-lean lens, zoom/pan, and the district+representative drill-down.
- data-model.md: stale '17,506 files' → 105,743.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/board.md

@@ -1,8 +1,13 @@
 # The Board
 
 `viz/board.html` is the Board — a rebuilt *view* of the canonical tree, never a
-source. It is a nationwide county choropleth (income / population / poverty
-lenses) where any county opens a drill-down of its officeholders and demographics.
+source. It is a nationwide county choropleth with four lenses — **income,
+population, poverty, and partisan lean** (the head-count D/R balance of each
+county's covering state + federal representatives) — that is **zoomable and
+pannable** (scroll to zoom 1×–16× toward the cursor, drag to pan, double-click to
+reset). Any county opens a drill-down of its officeholders, its demographics, and
+the **districts + representatives** that cover it (congressional, state senate,
+and state house, each with the share of the county it covers).
 
 ## Pipeline