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/sd/counties/marshall/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Marshall County, SD"
+classification: county
+fips: "46091"
+state: "SD"
+demographics:
+  population: 4344
+  population_under_18: 1218
+  population_18_64: 1062
+  population_65_plus: 2064
+  median_household_income: 78156
+  poverty_rate: 6.06
+  homeownership_rate: 80.31
+  unemployment_rate: 0.32
+  median_home_value: 156500
+  gini_index: 0.4487
+  vacancy_rate: 32.22
+  race_white: 3670
+  race_black: 19
+  race_asian: 8
+  race_native: 255
+  hispanic: 278
+  bachelors_plus: 929
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, sd]
+timestamp: "2026-07-03"
+---
+
+# Marshall County, SD
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 4344 |
+| Under 18 | 1218 |
+| 18–64 | 1062 |
+| 65+ | 2064 |
+| Median household income | 78156 |
+| Poverty rate | 6.06 |
+| Homeownership rate | 80.31 |
+| Unemployment rate | 0.32 |
+| Median home value | 156500 |
+| Gini index | 0.4487 |
+| Vacancy rate | 32.22 |
+| White | 3670 |
+| Black | 19 |
+| Asian | 8 |
+| Native | 255 |
+| Hispanic/Latino | 278 |
+| Bachelor's or higher | 929 |
+
+## Source
+
+- demographics: Census ACS 2023