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/seward/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Seward County, NE"
+classification: county
+fips: "31159"
+state: "NE"
+demographics:
+  population: 17641
+  population_under_18: 4111
+  population_18_64: 10316
+  population_65_plus: 3214
+  median_household_income: 82444
+  poverty_rate: 7.38
+  homeownership_rate: 72.01
+  unemployment_rate: 2.13
+  median_home_value: 241300
+  gini_index: 0.3907
+  vacancy_rate: 5.8
+  race_white: 16660
+  race_black: 144
+  race_asian: 94
+  race_native: 71
+  hispanic: 532
+  bachelors_plus: 4861
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ne]
+timestamp: "2026-07-03"
+---
+
+# Seward County, NE
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 17641 |
+| Under 18 | 4111 |
+| 18–64 | 10316 |
+| 65+ | 3214 |
+| Median household income | 82444 |
+| Poverty rate | 7.38 |
+| Homeownership rate | 72.01 |
+| Unemployment rate | 2.13 |
+| Median home value | 241300 |
+| Gini index | 0.3907 |
+| Vacancy rate | 5.8 |
+| White | 16660 |
+| Black | 144 |
+| Asian | 94 |
+| Native | 71 |
+| Hispanic/Latino | 532 |
+| Bachelor's or higher | 4861 |
+
+## Source
+
+- demographics: Census ACS 2023