fix(cables): preserve feature ownership across multipart geometry

Associate expanded cable and landing entities through a copied source index,
retain one reference stem per feature, and cover duplicate IDs, bundled
segments and frozen provider inputs with real Cesium regressions.

docs/CURRENT-STATE.md

@@ -5002,6 +5002,14 @@ three Cesium data sources; enabling rebuilds from the accepted parsed cache,
 and destroying clears it. Load ownership prevents cancelled work from adding
 entities after teardown.
 
+Multipart cable and landing-point geometry retains its source feature on every
+Cesium entity, including generated IDs and IDs that collide with Cesium's
+multipart suffixes. Each segment receives its own feature's styling, click
+destination and horizon-culling reference; each source feature has one reference
+stem. Source collections and their properties are not modified. The copied input
+adds `__gevSourceFeatureIndex`, which remains observable in Cesium properties
+and default descriptions.
+
 Natural Earth regions and neighborhood polygon lookup are package exports.
 Their existing lazy loaders, retry behavior, bundled datasets and attribution
 are unchanged. The cable dataset remains CC BY-NC-SA 3.0 and is not covered by