Municipals: nest 1,823 city officeholders under their real counties

Pull Atlas's place→county crosswalk (Census TIGER 2024 places × county
geometries, 32,041 places) into the build as a committed raw input, and
use it to resolve municipal officeholders whose county was unknown.

- data/place_county_crosswalk.jsonl — new raw input (place_geoid → county_geoid)
- build.py — place-name+state → county_geoid → ACS county slug as a fallback
  in county_slug(); city_slug() generalized to non-FL bare place labels
- data/sources/sources.yaml — provenance entry for the crosswalk

_unresolved municipals: 1919 → 58. The remainder are genuine: CT planning
regions (post-county), county-level rows mislabeled municipal upstream,
consolidated city-counties (null label), Wikidata-QID placeholder names,
and ambiguous same-name places — left unresolved rather than guessed.

Join key is county_geoid (5-digit FIPS); the directory slug is taken from
the ACS county node so municipals nest under the exact existing county dir.
Deterministic (byte-identical rebuild); validate.py passes 19,721 records.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

data/jurisdictions/us/states/mt/counties/gallatin/municipalities/belgrade/people/russell-c-nelson.md

@@ -0,0 +1,39 @@
+---
+type: Person
+title: "Russell C. Nelson"
+description: "Mayor, Belgrade, MT"
+role: "Mayor, Belgrade, MT"
+level: "municipal"
+branch: "executive"
+state: "MT"
+tenure:
+  start: "2003-08-01"
+  current: true
+ids:
+  person: "df798b47-fb1b-4fba-b711-a7979a7a3dbd"
+  office: "3607a57a-55f3-489b-985e-37e19cf2659a"
+  tenure: "398bf92b-c6df-4f75-b7e7-4efce0578854"
+  jurisdiction: "4d632739-b1d6-4612-bf01-9223301ff2aa"
+sources:
+  - field: office
+    source: "Wikidata"
+  - field: tenure
+    source: "Wikidata"
+  - field: jurisdiction
+    source: "Census TIGER 2024"
+confidence: official
+tags: [officeholder, municipal, executive, mt]
+timestamp: "2026-07-04"
+---
+
+# Russell C. Nelson
+
+Current Mayor, Belgrade, MT.
+
+## Sources
+
+- office: Wikidata
+- tenure: Wikidata
+- jurisdiction: Census TIGER 2024
+
+Generated from the Atlas officeholders v3 export (2026-07-04).