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/musselshell/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Musselshell County, MT"
+classification: county
+fips: "30065"
+state: "MT"
+demographics:
+  population: 5152
+  population_under_18: 998
+  population_18_64: 2660
+  population_65_plus: 1494
+  median_household_income: 58920
+  poverty_rate: 17.03
+  homeownership_rate: 85.06
+  unemployment_rate: 9.22
+  median_home_value: 235000
+  gini_index: 0.4739
+  vacancy_rate: 19.34
+  race_white: 4495
+  race_black: 0
+  race_asian: 0
+  race_native: 196
+  hispanic: 221
+  bachelors_plus: 1078
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, mt]
+timestamp: "2026-07-03"
+---
+
+# Musselshell County, MT
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 5152 |
+| Under 18 | 998 |
+| 18–64 | 2660 |
+| 65+ | 1494 |
+| Median household income | 58920 |
+| Poverty rate | 17.03 |
+| Homeownership rate | 85.06 |
+| Unemployment rate | 9.22 |
+| Median home value | 235000 |
+| Gini index | 0.4739 |
+| Vacancy rate | 19.34 |
+| White | 4495 |
+| Black | 0 |
+| Asian | 0 |
+| Native | 196 |
+| Hispanic/Latino | 221 |
+| Bachelor's or higher | 1078 |
+
+## Source
+
+- demographics: Census ACS 2023