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/mccone/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "McCone County, MT"
+classification: county
+fips: "30055"
+state: "MT"
+demographics:
+  population: 1670
+  population_under_18: 311
+  population_18_64: 851
+  population_65_plus: 508
+  median_household_income: 69375
+  poverty_rate: 5.04
+  homeownership_rate: 89.85
+  unemployment_rate: 2.13
+  median_home_value: 211600
+  gini_index: 0.3777
+  vacancy_rate: 26.25
+  race_white: 1538
+  race_black: 0
+  race_asian: 17
+  race_native: 42
+  hispanic: 4
+  bachelors_plus: 366
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, mt]
+timestamp: "2026-07-03"
+---
+
+# McCone County, MT
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 1670 |
+| Under 18 | 311 |
+| 18–64 | 851 |
+| 65+ | 508 |
+| Median household income | 69375 |
+| Poverty rate | 5.04 |
+| Homeownership rate | 89.85 |
+| Unemployment rate | 2.13 |
+| Median home value | 211600 |
+| Gini index | 0.3777 |
+| Vacancy rate | 26.25 |
+| White | 1538 |
+| Black | 0 |
+| Asian | 17 |
+| Native | 42 |
+| Hispanic/Latino | 4 |
+| Bachelor's or higher | 366 |
+
+## Source
+
+- demographics: Census ACS 2023