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/ca/counties/merced/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Merced County, CA"
+classification: county
+fips: "06047"
+state: "CA"
+demographics:
+  population: 290201
+  population_under_18: 83330
+  population_18_64: 172870
+  population_65_plus: 34001
+  median_household_income: 65510
+  poverty_rate: 19.09
+  homeownership_rate: 54.0
+  unemployment_rate: 10.56
+  median_home_value: 391800
+  gini_index: 0.4651
+  vacancy_rate: 5.3
+  race_white: 91462
+  race_black: 8082
+  race_asian: 20971
+  race_native: 4316
+  hispanic: 183933
+  bachelors_plus: 34910
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ca]
+timestamp: "2026-07-03"
+---
+
+# Merced County, CA
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 290201 |
+| Under 18 | 83330 |
+| 18–64 | 172870 |
+| 65+ | 34001 |
+| Median household income | 65510 |
+| Poverty rate | 19.09 |
+| Homeownership rate | 54.0 |
+| Unemployment rate | 10.56 |
+| Median home value | 391800 |
+| Gini index | 0.4651 |
+| Vacancy rate | 5.3 |
+| White | 91462 |
+| Black | 8082 |
+| Asian | 20971 |
+| Native | 4316 |
+| Hispanic/Latino | 183933 |
+| Bachelor's or higher | 34910 |
+
+## Source
+
+- demographics: Census ACS 2023