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/nd/counties/mercer/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Mercer County, ND"
+classification: county
+fips: "38057"
+state: "ND"
+demographics:
+  population: 8337
+  population_under_18: 1949
+  population_18_64: 4512
+  population_65_plus: 1876
+  median_household_income: 80946
+  poverty_rate: 9.67
+  homeownership_rate: 81.05
+  unemployment_rate: 2.53
+  median_home_value: 198500
+  gini_index: 0.3997
+  vacancy_rate: 22.99
+  race_white: 7647
+  race_black: 1
+  race_asian: 42
+  race_native: 204
+  hispanic: 231
+  bachelors_plus: 1680
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, nd]
+timestamp: "2026-07-03"
+---
+
+# Mercer County, ND
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 8337 |
+| Under 18 | 1949 |
+| 18–64 | 4512 |
+| 65+ | 1876 |
+| Median household income | 80946 |
+| Poverty rate | 9.67 |
+| Homeownership rate | 81.05 |
+| Unemployment rate | 2.53 |
+| Median home value | 198500 |
+| Gini index | 0.3997 |
+| Vacancy rate | 22.99 |
+| White | 7647 |
+| Black | 1 |
+| Asian | 42 |
+| Native | 204 |
+| Hispanic/Latino | 231 |
+| Bachelor's or higher | 1680 |
+
+## Source
+
+- demographics: Census ACS 2023