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/fl/counties/duval/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Duval County, FL"
+classification: county
+fips: "12031"
+state: "FL"
+demographics:
+  population: 1023153
+  population_under_18: 253220
+  population_18_64: 367532
+  population_65_plus: 402401
+  median_household_income: 71277
+  poverty_rate: 14.15
+  homeownership_rate: 58.23
+  unemployment_rate: 4.48
+  median_home_value: 303500
+  gini_index: 0.473
+  vacancy_rate: 8.71
+  race_white: 521574
+  race_black: 292981
+  race_asian: 50482
+  race_native: 2253
+  hispanic: 126467
+  bachelors_plus: 322765
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, fl]
+timestamp: "2026-07-03"
+---
+
+# Duval County, FL
+
+County jurisdiction — 80 officeholders mapped.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 1023153 |
+| Under 18 | 253220 |
+| 18–64 | 367532 |
+| 65+ | 402401 |
+| Median household income | 71277 |
+| Poverty rate | 14.15 |
+| Homeownership rate | 58.23 |
+| Unemployment rate | 4.48 |
+| Median home value | 303500 |
+| Gini index | 0.473 |
+| Vacancy rate | 8.71 |
+| White | 521574 |
+| Black | 292981 |
+| Asian | 50482 |
+| Native | 2253 |
+| Hispanic/Latino | 126467 |
+| Bachelor's or higher | 322765 |
+
+## Source
+
+- demographics: Census ACS 2023