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/mn/districts/03.md

@@ -0,0 +1,59 @@
+---
+type: Jurisdiction
+title: "MN-03"
+classification: congressional-district
+state: "MN"
+district: "MN-03"
+demographics:
+  population: 710658
+  population_under_18: 174530
+  population_18_64: 222188
+  population_65_plus: 313940
+  median_household_income: 106557
+  poverty_rate: 5.94
+  homeownership_rate: 71.34
+  unemployment_rate: 3.51
+  median_home_value: 423800
+  gini_index: 0.4678
+  vacancy_rate: 3.72
+  race_white: 496834
+  race_black: 76704
+  race_asian: 63331
+  hispanic: 34877
+  bachelors_plus: 267265
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, congressional-district, mn]
+timestamp: "2026-07-03"
+---
+
+# MN-03
+
+Congressional district MN-03.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 710658 |
+| Under 18 | 174530 |
+| 18–64 | 222188 |
+| 65+ | 313940 |
+| Median household income | 106557 |
+| Poverty rate | 5.94 |
+| Homeownership rate | 71.34 |
+| Unemployment rate | 3.51 |
+| Median home value | 423800 |
+| Gini index | 0.4678 |
+| Vacancy rate | 3.72 |
+| White | 496834 |
+| Black | 76704 |
+| Asian | 63331 |
+| Hispanic/Latino | 34877 |
+| Bachelor's or higher | 267265 |
+
+## Source
+
+- demographics: Census ACS 2023