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/tx/counties/dimmit/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Dimmit County, TX"
+classification: county
+fips: "48127"
+state: "TX"
+demographics:
+  population: 8380
+  population_under_18: 2178
+  population_18_64: 4844
+  population_65_plus: 1358
+  median_household_income: 38808
+  poverty_rate: 41.11
+  homeownership_rate: 66.54
+  unemployment_rate: 8.42
+  median_home_value: 85500
+  gini_index: 0.5213
+  vacancy_rate: 25.48
+  race_white: 1203
+  race_black: 94
+  race_asian: 46
+  race_native: 234
+  hispanic: 7303
+  bachelors_plus: 661
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, tx]
+timestamp: "2026-07-03"
+---
+
+# Dimmit County, TX
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 8380 |
+| Under 18 | 2178 |
+| 18–64 | 4844 |
+| 65+ | 1358 |
+| Median household income | 38808 |
+| Poverty rate | 41.11 |
+| Homeownership rate | 66.54 |
+| Unemployment rate | 8.42 |
+| Median home value | 85500 |
+| Gini index | 0.5213 |
+| Vacancy rate | 25.48 |
+| White | 1203 |
+| Black | 94 |
+| Asian | 46 |
+| Native | 234 |
+| Hispanic/Latino | 7303 |
+| Bachelor's or higher | 661 |
+
+## Source
+
+- demographics: Census ACS 2023