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/ca/counties/marin/index.md

@@ -32,7 +32,7 @@ timestamp: "2026-07-03"
 
 # Marin County, CA
 
-County jurisdiction.
+County jurisdiction — 1 officeholders mapped.
 
 ## Demographics (ACS 2023)
 

data/jurisdictions/us/states/ca/counties/marin/municipalities/mill-valley/people/john-mccauley.md

@@ -0,0 +1,39 @@
+---
+type: Person
+title: "John McCauley"
+description: "Mayor, Mill Valley, CA"
+role: "Mayor, Mill Valley, CA"
+level: "municipal"
+branch: "executive"
+state: "CA"
+tenure:
+  start: "2013-11-01"
+  current: true
+ids:
+  person: "59a744a2-9389-48d6-a8fb-e31d6d7eaf51"
+  office: "d12a3d3f-ebf2-4220-b8d0-582e3f9f7b36"
+  tenure: "090c7ad5-032e-47ca-af8b-6845b8c731a3"
+  jurisdiction: "51f8e02a-e968-4293-8beb-e8adc3942b5c"
+sources:
+  - field: office
+    source: "Wikidata"
+  - field: tenure
+    source: "Wikidata"
+  - field: jurisdiction
+    source: "Census TIGER 2024"
+confidence: official
+tags: [officeholder, municipal, executive, ca]
+timestamp: "2026-07-04"
+---
+
+# John McCauley
+
+Current Mayor, Mill Valley, CA.
+
+## Sources
+
+- office: Wikidata
+- tenure: Wikidata
+- jurisdiction: Census TIGER 2024
+
+Generated from the Atlas officeholders v3 export (2026-07-04).