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/az/districts/04.md

@@ -0,0 +1,59 @@
+---
+type: Jurisdiction
+title: "AZ-04"
+classification: congressional-district
+state: "AZ"
+district: "AZ-04"
+demographics:
+  population: 793264
+  population_under_18: 171375
+  population_18_64: 316215
+  population_65_plus: 305674
+  median_household_income: 82539
+  poverty_rate: 12.09
+  homeownership_rate: 55.6
+  unemployment_rate: 4.38
+  median_home_value: 449900
+  gini_index: 0.4412
+  vacancy_rate: 8.87
+  race_white: 470113
+  race_black: 37046
+  race_asian: 55026
+  hispanic: 206661
+  bachelors_plus: 228151
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, congressional-district, az]
+timestamp: "2026-07-03"
+---
+
+# AZ-04
+
+Congressional district AZ-04.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 793264 |
+| Under 18 | 171375 |
+| 18–64 | 316215 |
+| 65+ | 305674 |
+| Median household income | 82539 |
+| Poverty rate | 12.09 |
+| Homeownership rate | 55.6 |
+| Unemployment rate | 4.38 |
+| Median home value | 449900 |
+| Gini index | 0.4412 |
+| Vacancy rate | 8.87 |
+| White | 470113 |
+| Black | 37046 |
+| Asian | 55026 |
+| Hispanic/Latino | 206661 |
+| Bachelor's or higher | 228151 |
+
+## Source
+
+- demographics: Census ACS 2023