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/sc/districts/04.md

@@ -0,0 +1,59 @@
+---
+type: Jurisdiction
+title: "SC-04"
+classification: congressional-district
+state: "SC"
+district: "SC-04"
+demographics:
+  population: 810387
+  population_under_18: 203356
+  population_18_64: 274400
+  population_65_plus: 332631
+  median_household_income: 78299
+  poverty_rate: 10.73
+  homeownership_rate: 68.44
+  unemployment_rate: 4.23
+  median_home_value: 309800
+  gini_index: 0.4476
+  vacancy_rate: 7.13
+  race_white: 529805
+  race_black: 136514
+  race_asian: 23101
+  hispanic: 99462
+  bachelors_plus: 205311
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, congressional-district, sc]
+timestamp: "2026-07-03"
+---
+
+# SC-04
+
+Congressional district SC-04.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 810387 |
+| Under 18 | 203356 |
+| 18–64 | 274400 |
+| 65+ | 332631 |
+| Median household income | 78299 |
+| Poverty rate | 10.73 |
+| Homeownership rate | 68.44 |
+| Unemployment rate | 4.23 |
+| Median home value | 309800 |
+| Gini index | 0.4476 |
+| Vacancy rate | 7.13 |
+| White | 529805 |
+| Black | 136514 |
+| Asian | 23101 |
+| Hispanic/Latino | 99462 |
+| Bachelor's or higher | 205311 |
+
+## Source
+
+- demographics: Census ACS 2023