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/wv/counties/summers/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Summers County, WV"
+classification: county
+fips: "54089"
+state: "WV"
+demographics:
+  population: 11729
+  population_under_18: 1838
+  population_18_64: 6709
+  population_65_plus: 3182
+  median_household_income: 40699
+  poverty_rate: 23.79
+  homeownership_rate: 81.15
+  unemployment_rate: 2.2
+  median_home_value: 140100
+  gini_index: 0.4386
+  vacancy_rate: 28.45
+  race_white: 10651
+  race_black: 718
+  race_asian: 0
+  race_native: 27
+  hispanic: 157
+  bachelors_plus: 2460
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, wv]
+timestamp: "2026-07-03"
+---
+
+# Summers County, WV
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 11729 |
+| Under 18 | 1838 |
+| 18–64 | 6709 |
+| 65+ | 3182 |
+| Median household income | 40699 |
+| Poverty rate | 23.79 |
+| Homeownership rate | 81.15 |
+| Unemployment rate | 2.2 |
+| Median home value | 140100 |
+| Gini index | 0.4386 |
+| Vacancy rate | 28.45 |
+| White | 10651 |
+| Black | 718 |
+| Asian | 0 |
+| Native | 27 |
+| Hispanic/Latino | 157 |
+| Bachelor's or higher | 2460 |
+
+## Source
+
+- demographics: Census ACS 2023