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/wi/counties/walworth/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Walworth County, WI"
+classification: county
+fips: "55127"
+state: "WI"
+demographics:
+  population: 105657
+  population_under_18: 20610
+  population_18_64: 63952
+  population_65_plus: 21095
+  median_household_income: 80520
+  poverty_rate: 9.36
+  homeownership_rate: 70.63
+  unemployment_rate: 2.88
+  median_home_value: 302700
+  gini_index: 0.452
+  vacancy_rate: 20.39
+  race_white: 90011
+  race_black: 853
+  race_asian: 916
+  race_native: 392
+  hispanic: 12821
+  bachelors_plus: 31136
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, wi]
+timestamp: "2026-07-03"
+---
+
+# Walworth County, WI
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 105657 |
+| Under 18 | 20610 |
+| 18–64 | 63952 |
+| 65+ | 21095 |
+| Median household income | 80520 |
+| Poverty rate | 9.36 |
+| Homeownership rate | 70.63 |
+| Unemployment rate | 2.88 |
+| Median home value | 302700 |
+| Gini index | 0.452 |
+| Vacancy rate | 20.39 |
+| White | 90011 |
+| Black | 853 |
+| Asian | 916 |
+| Native | 392 |
+| Hispanic/Latino | 12821 |
+| Bachelor's or higher | 31136 |
+
+## Source
+
+- demographics: Census ACS 2023