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/nc/counties/davidson/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Davidson County, NC"
+classification: county
+fips: "37057"
+state: "NC"
+demographics:
+  population: 172954
+  population_under_18: 37476
+  population_18_64: 102580
+  population_65_plus: 32898
+  median_household_income: 64172
+  poverty_rate: 12.85
+  homeownership_rate: 74.37
+  unemployment_rate: 4.08
+  median_home_value: 219100
+  gini_index: 0.441
+  vacancy_rate: 8.57
+  race_white: 133972
+  race_black: 16096
+  race_asian: 2667
+  race_native: 373
+  hispanic: 15456
+  bachelors_plus: 34779
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, nc]
+timestamp: "2026-07-03"
+---
+
+# Davidson County, NC
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 172954 |
+| Under 18 | 37476 |
+| 18–64 | 102580 |
+| 65+ | 32898 |
+| Median household income | 64172 |
+| Poverty rate | 12.85 |
+| Homeownership rate | 74.37 |
+| Unemployment rate | 4.08 |
+| Median home value | 219100 |
+| Gini index | 0.441 |
+| Vacancy rate | 8.57 |
+| White | 133972 |
+| Black | 16096 |
+| Asian | 2667 |
+| Native | 373 |
+| Hispanic/Latino | 15456 |
+| Bachelor's or higher | 34779 |
+
+## Source
+
+- demographics: Census ACS 2023