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/ca/counties/san-joaquin/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "San Joaquin County, CA"
+classification: county
+fips: "06077"
+state: "CA"
+demographics:
+  population: 797334
+  population_under_18: 208824
+  population_18_64: 480780
+  population_65_plus: 107730
+  median_household_income: 92179
+  poverty_rate: 12.41
+  homeownership_rate: 61.99
+  unemployment_rate: 7.57
+  median_home_value: 530700
+  gini_index: 0.4375
+  vacancy_rate: 5.73
+  race_white: 254518
+  race_black: 54932
+  race_asian: 148289
+  race_native: 11882
+  hispanic: 339161
+  bachelors_plus: 154334
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ca]
+timestamp: "2026-07-03"
+---
+
+# San Joaquin County, CA
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 797334 |
+| Under 18 | 208824 |
+| 18–64 | 480780 |
+| 65+ | 107730 |
+| Median household income | 92179 |
+| Poverty rate | 12.41 |
+| Homeownership rate | 61.99 |
+| Unemployment rate | 7.57 |
+| Median home value | 530700 |
+| Gini index | 0.4375 |
+| Vacancy rate | 5.73 |
+| White | 254518 |
+| Black | 54932 |
+| Asian | 148289 |
+| Native | 11882 |
+| Hispanic/Latino | 339161 |
+| Bachelor's or higher | 154334 |
+
+## Source
+
+- demographics: Census ACS 2023