feat(osm-equity): uniform 2-sq-mi tile units + areal equity interpolation

[rlemke]

Breaks a city into a regular grid of ~2-square-mile tiles as an alternative to irregular census tracts:

• build_tiles(bbox, sqmi=2.0) — uniform square-tile units.
• areal_interpolate_equity — dasymetric intersection-area weighting of ACS tract equity onto tiles (tiles over no populated tract omitted, not zeroed).

Metrics + heat map work unchanged on tiles. On real SF data, 2-sq-mi tiling gives 45 tiles and raises Global Moran's I of attribute completeness from 0.33 (tracts) → 0.48 (tiles) — a cleaner spatial signal (less MAUP noise). 3 offline unit tests; 16 offline tests pass; ruff clean.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces a regular square-tile grid analysis option as an alternative to irregular census tracts, adding functions to build tiles and perform dasymetric areal interpolation of equity metrics, along with corresponding unit tests and documentation. The reviewer feedback suggests enhancing robustness by adding input validation for tile generation parameters, implementing guard checks for empty inputs, and handling potential topological errors and missing dictionary keys during spatial intersection calculations.

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[gemini-code-assist]

Add defensive validation for the sqmi parameter and the bounding box coordinates to prevent potential division-by-zero errors or invalid grid generation.

Suggested change

	xmin, ymin, xmax, ymax = bbox
	xmin, ymin, xmax, ymax = bbox
	if sqmi <= 0:
	raise ValueError("sqmi must be a positive number.")
	if xmin >= xmax or ymin >= ymax:
	raise ValueError("Invalid bounding box: xmin must be < xmax and ymin must be < ymax.")

[gemini-code-assist]

Add a guard check at the beginning of areal_interpolate_equity to handle empty inputs gracefully and avoid unnecessary processing or potential issues with empty STRtree initialization.

Suggested change

	from shapely.strtree import STRtree
	if not tiles or not tract_records:
	return {}
	from shapely.strtree import STRtree

[gemini-code-assist]

Improve robustness by using safe dictionary access for the equity profile and its fields, and handle potential topological errors (e.g., self-intersections in real-world TIGERweb data) during spatial intersection using a fallback .buffer(0).

Suggested change

	eq = tract_records[idx]["equity"]
	if eq["median_income"] <= 0:
	continue
	inter = poly.intersection(geoms[idx])
	w = inter.area
	if w <= 0:
	continue
	for f in _EQ_FIELDS:
	acc[f] += eq[f] * w
	wsum += w
	eq = tract_records[idx].get("equity")
	if not eq or eq.get("median_income", 0) <= 0:
	continue
	try:
	inter = poly.intersection(geoms[idx])
	except Exception:
	try:
	inter = poly.buffer(0).intersection(geoms[idx].buffer(0))
	except Exception:
	continue
	w = inter.area
	if w <= 0:
	continue
	for f in _EQ_FIELDS:
	acc[f] += eq.get(f, 0.0) * w
	wsum += w