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/tn/counties/shelby/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Shelby County, TN"
+classification: county
+fips: "47157"
+state: "TN"
+demographics:
+  population: 919173
+  population_under_18: 233802
+  population_18_64: 549321
+  population_65_plus: 136050
+  median_household_income: 63767
+  poverty_rate: 17.82
+  homeownership_rate: 54.14
+  unemployment_rate: 7.2
+  median_home_value: 249100
+  gini_index: 0.513
+  vacancy_rate: 10.29
+  race_white: 312512
+  race_black: 482006
+  race_asian: 27121
+  race_native: 3475
+  hispanic: 81334
+  bachelors_plus: 292293
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, tn]
+timestamp: "2026-07-03"
+---
+
+# Shelby County, TN
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 919173 |
+| Under 18 | 233802 |
+| 18–64 | 549321 |
+| 65+ | 136050 |
+| Median household income | 63767 |
+| Poverty rate | 17.82 |
+| Homeownership rate | 54.14 |
+| Unemployment rate | 7.2 |
+| Median home value | 249100 |
+| Gini index | 0.513 |
+| Vacancy rate | 10.29 |
+| White | 312512 |
+| Black | 482006 |
+| Asian | 27121 |
+| Native | 3475 |
+| Hispanic/Latino | 81334 |
+| Bachelor's or higher | 292293 |
+
+## Source
+
+- demographics: Census ACS 2023