Fix spatial docs: real Census data, chart alignment, pin mkdocs <2 (#307)

* Fix spatial docs: real Census data, chart alignment, pin mkdocs <2

- Replace fake/random dot density data with real ACS race/ethnicity data
  for Alameda County via pypums.get_acs() + as_dot_density()
- Fix projection misalignment by deriving county boundary from dissolved
  tract geometries (inline GeoJSON) instead of external topojson
- Update all output blocks (get_acs, attach_geometry, dot density) to
  match actual Census API responses (values, column order, NAME format)
- Fix choropleth legend formats: population "~s" (10M), income "$~s" ($50k)
- Pin mkdocs>=1.6,<2 to prevent incompatible mkdocs 2.x installation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Address PR review: extract inline data, fix projection and caption

- Move 12K lines of inline dot data to docs/assets/alameda-dots.json
  (spatial.md drops from 13K to 1K lines)
- Keep county boundary inline (small, avoids projection auto-fit issue)
- Fix projection mismatch: code example now uses mercator to match chart
- Clarify dots_per_value difference between code (100) and preview (500)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Use dots_per_value=500 in code example to match preview chart

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Externalize boundary data to alameda-boundary.json

Replace 250 lines of inline boundary coordinates with URL reference.
spatial.md drops from 1,254 to 1,008 lines.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: chekos

docs/assets/alameda-boundary.json

@@ -0,0 +1 @@
+[{"id": "0", "type": "Feature", "properties": {}, "geometry": {"type": "Polygon", "coordinates": [[[-122.279401, 37.768134], [-122.289012, 37.764608], [-122.328024583922, 37.7813728248168], [-122.342253, 37.805558], [-122.317296, 37.816261], [-122.330369, 37.82058], [-122.296618, 37.828465], [-122.298044, 37.836349], [-122.315913, 37.836812], [-122.314195, 37.842311], [-122.299233, 37.840479], [-122.301372, 37.855493], [-122.308352, 37.862934], [-122.316944, 37.858809], [-122.325193, 37.874276], [-122.308502, 37.87088], [-122.326912, 37.892492], [-122.311101, 37.89043], [-122.313147550967, 37.8972880379388], [-122.27108, 37.905824], [-122.241971, 37.881927], [-122.22362, 37.878566], [-122.208898, 37.851941], [-122.185116, 37.837353], [-122.185577, 37.820726], [-122.166873, 37.813424], [-122.157392, 37.817952], [-122.140317, 37.804701], [-122.045473, 37.798126], [-121.997411, 37.76336], [-122.011771, 37.747428], [-121.96004, 37.718531], [-121.556997, 37.816488], [-121.556655, 37.542732], [-121.541833, 37.530215], [-121.501475, 37.525003], [-121.496017, 37.504938], [-121.482065, 37.501439], [-121.47106, 37.483317], [-121.865267, 37.484637], [-121.925041, 37.454186], [-121.944914, 37.469163], [-121.951921, 37.461461], [-122.040872722043, 37.4628633674882], [-122.053431, 37.473321], [-122.055541, 37.49523], [-122.10942, 37.500254], [-122.111998, 37.528851], [-122.147014, 37.588411], [-122.152993, 37.641923], [-122.167587, 37.677178], [-122.204161, 37.711603], [-122.214744, 37.699247], [-122.252172, 37.724952], [-122.261679, 37.743373], [-122.243947, 37.751779], [-122.279401, 37.768134]]]}, "bbox": [-122.342253, 37.454186, -121.47106, 37.905824]}]