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/mt/counties/powder-river/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Powder River County, MT"
+classification: county
+fips: "30075"
+state: "MT"
+demographics:
+  population: 1815
+  population_under_18: 364
+  population_18_64: 906
+  population_65_plus: 545
+  median_household_income: 67045
+  poverty_rate: 8.12
+  homeownership_rate: 78.81
+  unemployment_rate: 0.71
+  median_home_value: 192100
+  gini_index: 0.4717
+  vacancy_rate: 27.16
+  race_white: 1642
+  race_black: 0
+  race_asian: 17
+  race_native: 40
+  hispanic: 26
+  bachelors_plus: 488
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, mt]
+timestamp: "2026-07-03"
+---
+
+# Powder River County, MT
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 1815 |
+| Under 18 | 364 |
+| 18–64 | 906 |
+| 65+ | 545 |
+| Median household income | 67045 |
+| Poverty rate | 8.12 |
+| Homeownership rate | 78.81 |
+| Unemployment rate | 0.71 |
+| Median home value | 192100 |
+| Gini index | 0.4717 |
+| Vacancy rate | 27.16 |
+| White | 1642 |
+| Black | 0 |
+| Asian | 17 |
+| Native | 40 |
+| Hispanic/Latino | 26 |
+| Bachelor's or higher | 488 |
+
+## Source
+
+- demographics: Census ACS 2023