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/mo/districts/07.md

@@ -0,0 +1,59 @@
+---
+type: Jurisdiction
+title: "MO-07"
+classification: congressional-district
+state: "MO"
+district: "MO-07"
+demographics:
+  population: 803037
+  population_under_18: 198979
+  population_18_64: 267463
+  population_65_plus: 336595
+  median_household_income: 66064
+  poverty_rate: 11.94
+  homeownership_rate: 66.08
+  unemployment_rate: 3.49
+  median_home_value: 242400
+  gini_index: 0.4517
+  vacancy_rate: 10.72
+  race_white: 686687
+  race_black: 15684
+  race_asian: 11154
+  hispanic: 54833
+  bachelors_plus: 158195
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, congressional-district, mo]
+timestamp: "2026-07-03"
+---
+
+# MO-07
+
+Congressional district MO-07.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 803037 |
+| Under 18 | 198979 |
+| 18–64 | 267463 |
+| 65+ | 336595 |
+| Median household income | 66064 |
+| Poverty rate | 11.94 |
+| Homeownership rate | 66.08 |
+| Unemployment rate | 3.49 |
+| Median home value | 242400 |
+| Gini index | 0.4517 |
+| Vacancy rate | 10.72 |
+| White | 686687 |
+| Black | 15684 |
+| Asian | 11154 |
+| Hispanic/Latino | 54833 |
+| Bachelor's or higher | 158195 |
+
+## Source
+
+- demographics: Census ACS 2023