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/keith/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Keith County, NE"
+classification: county
+fips: "31101"
+state: "NE"
+demographics:
+  population: 8219
+  population_under_18: 1692
+  population_18_64: 4233
+  population_65_plus: 2294
+  median_household_income: 60645
+  poverty_rate: 10.37
+  homeownership_rate: 71.12
+  unemployment_rate: 2.03
+  median_home_value: 168000
+  gini_index: 0.4209
+  vacancy_rate: 25.46
+  race_white: 7190
+  race_black: 91
+  race_asian: 59
+  race_native: 20
+  hispanic: 749
+  bachelors_plus: 1766
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ne]
+timestamp: "2026-07-03"
+---
+
+# Keith County, NE
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 8219 |
+| Under 18 | 1692 |
+| 18–64 | 4233 |
+| 65+ | 2294 |
+| Median household income | 60645 |
+| Poverty rate | 10.37 |
+| Homeownership rate | 71.12 |
+| Unemployment rate | 2.03 |
+| Median home value | 168000 |
+| Gini index | 0.4209 |
+| Vacancy rate | 25.46 |
+| White | 7190 |
+| Black | 91 |
+| Asian | 59 |
+| Native | 20 |
+| Hispanic/Latino | 749 |
+| Bachelor's or higher | 1766 |
+
+## Source
+
+- demographics: Census ACS 2023