Assign fallback unique ID to region features after duplicate id removal

[napakalas]

When processing features with a region property, the id is intentionally removed to prevent duplicate IDs:

flatmap-maker/mapmaker/flatmap/layers.py

Lines 344 to 347 in 091901a

	# So that any region doesn't have a duplicate id
	region_properties.pop('id', None)
	region_properties.pop('userdata', None)
	region = self.flatmap.new_feature(self.id, feature.geometry.representative_point(), region_properties)

However, this results in region features having no identifier at all. For example, UBERON:0001161 (body of stomach)

This impacts path handling. For example, portal/2 passes through UBERON:0001161, but without an id, the system cannot determine whether the region is present or associated with the path.

Proposed Solution
Assign a fallback unique ID (e.g., based on featureId or UBERON ID or CLASS ID) when adding region features to layer_features.
This preserves the original intent (no duplicate IDs) while ensuring regions remain identifiable for downstream logic.

Missing id for UBERON:0001161:

"868": {
    "class": "digestive_18",
    "tile-layer": "features",
    "name": "body of stomach",
    "label": "body of stomach",
    "minzoom": 0,
    "featureId": 868,
    "models": "UBERON:0001161",
    "bounds": [
        3.1202072103334273,
        -3.957220372623511,
        4.266365794272358,
        -2.1002029334923114
    ],
    "markerPosition": [
        3.751318297275743,
        -2.9706798580850613
    ],
    "geometry": "Polygon",
    "layer": "whole-rat",
    "anatomical-nodes": [
        "[\"UBERON:0001161\", []]"
    ]
}