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/districts/07.md

@@ -0,0 +1,59 @@
+---
+type: Jurisdiction
+title: "MA-07"
+classification: congressional-district
+state: "MA"
+district: "MA-07"
+demographics:
+  population: 789900
+  population_under_18: 160778
+  population_18_64: 377852
+  population_65_plus: 251270
+  median_household_income: 98603
+  poverty_rate: 16.35
+  homeownership_rate: 34.21
+  unemployment_rate: 4.36
+  median_home_value: 687400
+  gini_index: 0.5006
+  vacancy_rate: 6.42
+  race_white: 320490
+  race_black: 155669
+  race_asian: 91046
+  hispanic: 183358
+  bachelors_plus: 272970
+sources:
+  - field: demographics
+    source: Census ACS 2023
+confidence: official
+tags: [jurisdiction, congressional-district, ma]
+timestamp: "2026-07-03"
+---
+
+# MA-07
+
+Congressional district MA-07.
+
+## Demographics (ACS 2023)
+
+| Measure | Value |
+| --- | --- |
+| Population | 789900 |
+| Under 18 | 160778 |
+| 18–64 | 377852 |
+| 65+ | 251270 |
+| Median household income | 98603 |
+| Poverty rate | 16.35 |
+| Homeownership rate | 34.21 |
+| Unemployment rate | 4.36 |
+| Median home value | 687400 |
+| Gini index | 0.5006 |
+| Vacancy rate | 6.42 |
+| White | 320490 |
+| Black | 155669 |
+| Asian | 91046 |
+| Hispanic/Latino | 183358 |
+| Bachelor's or higher | 272970 |
+
+## Source
+
+- demographics: Census ACS 2023