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/ma/counties/middlesex/index.md

@@ -0,0 +1,61 @@
+---
+type: Jurisdiction
+title: "Middlesex County, MA"
+classification: county
+fips: "25017"
+state: "MA"
+demographics:
+  population: 1638365
+  population_under_18: 318865
+  population_18_64: 1052493
+  population_65_plus: 267007
+  median_household_income: 130847
+  poverty_rate: 7.74
+  homeownership_rate: 61.32
+  unemployment_rate: 4.35
+  median_home_value: 727800
+  gini_index: 0.4747
+  vacancy_rate: 4.52
+  race_white: 1093628
+  race_black: 80327
+  race_asian: 223582
+  race_native: 3246
+  hispanic: 151761
+  bachelors_plus: 1044737
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, county, ma]
+timestamp: "2026-07-03"
+---
+
+# Middlesex County, MA
+
+County jurisdiction.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 1638365 |
+| Under 18 | 318865 |
+| 18–64 | 1052493 |
+| 65+ | 267007 |
+| Median household income | 130847 |
+| Poverty rate | 7.74 |
+| Homeownership rate | 61.32 |
+| Unemployment rate | 4.35 |
+| Median home value | 727800 |
+| Gini index | 0.4747 |
+| Vacancy rate | 4.52 |
+| White | 1093628 |
+| Black | 80327 |
+| Asian | 223582 |
+| Native | 3246 |
+| Hispanic/Latino | 151761 |
+| Bachelor's or higher | 1044737 |
+
+## Source
+
+- demographics: Census ACS 2023