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/al/counties/covington/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Covington County, AL"
+classification: county
+fips: "01039"
+state: "AL"
+demographics:
+  population: 37628
+  population_under_18: 8346
+  population_18_64: 20927
+  population_65_plus: 8355
+  median_household_income: 52456
+  poverty_rate: 19.19
+  homeownership_rate: 76.27
+  unemployment_rate: 5.17
+  median_home_value: 152100
+  gini_index: 0.4876
+  vacancy_rate: 21.76
+  race_white: 31206
+  race_black: 4497
+  race_asian: 173
+  race_native: 117
+  hispanic: 765
+  bachelors_plus: 6474
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, al]
+timestamp: "2026-07-03"
+---
+
+# Covington County, AL
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 37628 |
+| Under 18 | 8346 |
+| 18–64 | 20927 |
+| 65+ | 8355 |
+| Median household income | 52456 |
+| Poverty rate | 19.19 |
+| Homeownership rate | 76.27 |
+| Unemployment rate | 5.17 |
+| Median home value | 152100 |
+| Gini index | 0.4876 |
+| Vacancy rate | 21.76 |
+| White | 31206 |
+| Black | 4497 |
+| Asian | 173 |
+| Native | 117 |
+| Hispanic/Latino | 765 |
+| Bachelor's or higher | 6474 |
+
+## Source
+
+- demographics: Census ACS 2023