Docs: document the Board pipeline + Artifact redeploy step

Add `make board` and docs/board.md so the Board's rebuild-and-redeploy flow
is a documented step, not tribal knowledge. Covers why the county data is
inlined (self-contained Artifact / CSP), that build_viz.py injects it into
board.html, and that a live Artifact must be REPUBLISHED (editing the repo
file doesn't update a deployed Artifact). README links it in both the make
list and the docs index.

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

docs/board.md

@@ -0,0 +1,55 @@
+# 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.
+
+## Pipeline
+
+```text
+data/officeholders-v3.jsonl ─┐
+data/acs_county.jsonl        ├─ scripts/build_viz.py ─> viz/board.html (inlined)
+data/place_county_crosswalk  ┤                        └> viz/county_*.json (companions)
+data/fec_candidates, acs_cd ─┘
+```
+
+`build_viz.py` resolves each official to a county with the **same**
+`build.place_resolver` + `county_slug` the tree uses, so the Board and the tree
+agree by construction. It then does two things:
+
+1. Writes `viz/county_data.json` and `viz/county_detail.json` — human-readable,
+   diffable companions.
+2. **Injects** the fresh data straight into `board.html`, rewriting three marked
+   lines: `const D` (per-county summary), `const CDETAIL` (per-county rosters),
+   and `const TOTALS` (the header counts).
+
+## Why the data is inlined
+
+The Board is deployed as a **self-contained Artifact**, and the Artifact CSP
+blocks every external request — no `fetch`, no external scripts, styles, or
+fonts. So the county data cannot be loaded at runtime; it must live inside
+`board.html`. That is why `build_viz.py` writes into the file rather than letting
+the page read the JSON companions (nothing reads them — they exist for diffs).
+
+The header totals (officeholders, candidates, counties, districts) read from the
+injected `const TOTALS` object — never hardcoded — so they cannot drift out of
+sync on the next rebuild.
+
+## Rebuild
+
+```bash
+make board       # regenerate board.html + the JSON companions from the tree
+```
+
+`board.html` is deterministic: two runs are byte-identical.
+
+## Redeploy the Artifact
+
+Editing `board.html` in the repo does **not** update a live Artifact — the
+deployed copy is a snapshot. After `make board`, the Board must be **republished**
+to its existing Artifact URL for the change to appear on claude.ai.
+
+Republishing to the *same* URL requires passing that URL back to the publish step
+(`Artifact url=<existing-url>`); publishing without it mints a new URL. Keep the
+current Board Artifact URL in the team's deployment record so redeploys stay in
+place instead of scattering new links.