Add elections + demographics layer: candidates and jurisdictions

Two new entity types, built deterministically from the raw exports:

  Candidate (2,494)     — 2026 federal candidates as files under
                          us/states/<st>/candidates/, with office,
                          district, party, incumbent/challenger stance,
                          campaign committee, FEC id.

  Jurisdiction (3,494)  — demographic nodes carrying ACS 2023 data:
                          3,131 counties (deduped from 3,231 mixed-format
                          fips keys) + 363 congressional districts.

This completes the county skeleton nationwide: every US county is now a
browsable node with population, income, poverty, race, education and
housing — even where no officeholders are mapped yet. In Florida the
county index.md merges into the existing dir and reports its officeholder
count alongside demographics (St. Lucie: 33 officeholders + ACS profile).

build.py now reads seven raw inputs in one deterministic pass
(byte-identical on rerun). The mirror now answers: who holds power, who
runs this institution, who is running, and what each place is made of.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

data/jurisdictions/us/states/ne/counties/box-butte/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Box Butte County, NE"
+classification: county
+fips: "31013"
+state: "NE"
+demographics:
+  population: 10714
+  population_under_18: 2697
+  population_18_64: 5790
+  population_65_plus: 2227
+  median_household_income: 68892
+  poverty_rate: 10.79
+  homeownership_rate: 76.83
+  unemployment_rate: 1.6
+  median_home_value: 166600
+  gini_index: 0.3947
+  vacancy_rate: 13.56
+  race_white: 8735
+  race_black: 15
+  race_asian: 43
+  race_native: 338
+  hispanic: 1352
+  bachelors_plus: 1828
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ne]
+timestamp: "2026-07-03"
+---
+
+# Box Butte County, NE
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 10714 |
+| Under 18 | 2697 |
+| 18–64 | 5790 |
+| 65+ | 2227 |
+| Median household income | 68892 |
+| Poverty rate | 10.79 |
+| Homeownership rate | 76.83 |
+| Unemployment rate | 1.6 |
+| Median home value | 166600 |
+| Gini index | 0.3947 |
+| Vacancy rate | 13.56 |
+| White | 8735 |
+| Black | 15 |
+| Asian | 43 |
+| Native | 338 |
+| Hispanic/Latino | 1352 |
+| Bachelor's or higher | 1828 |
+
+## Source
+
+- demographics: Census ACS 2023