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/ky/counties/mercer/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Mercer County, KY"
+classification: county
+fips: "21167"
+state: "KY"
+demographics:
+  population: 23028
+  population_under_18: 5017
+  population_18_64: 13648
+  population_65_plus: 4363
+  median_household_income: 64824
+  poverty_rate: 10.66
+  homeownership_rate: 75.93
+  unemployment_rate: 4.36
+  median_home_value: 202300
+  gini_index: 0.415
+  vacancy_rate: 12.33
+  race_white: 20783
+  race_black: 743
+  race_asian: 43
+  race_native: 43
+  hispanic: 779
+  bachelors_plus: 5487
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ky]
+timestamp: "2026-07-03"
+---
+
+# Mercer County, KY
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 23028 |
+| Under 18 | 5017 |
+| 18–64 | 13648 |
+| 65+ | 4363 |
+| Median household income | 64824 |
+| Poverty rate | 10.66 |
+| Homeownership rate | 75.93 |
+| Unemployment rate | 4.36 |
+| Median home value | 202300 |
+| Gini index | 0.415 |
+| Vacancy rate | 12.33 |
+| White | 20783 |
+| Black | 743 |
+| Asian | 43 |
+| Native | 43 |
+| Hispanic/Latino | 779 |
+| Bachelor's or higher | 5487 |
+
+## Source
+
+- demographics: Census ACS 2023