FastAlphaTriangulationException

Thrown by FastAlphaHomologyContext when HelixDelaunay's own triangulation does not satisfy the "every facet has 1 or 2 containing top simplices" precondition this engine's dual graph needs (see the class doc's own "new finding" section) -- a real but rare (~1-in-18700 measured, ambient dimension 2) HelixDelaunay limitation, not a sign the input is malformed or that its persistent homology is somehow uncomputable. Deliberately a distinct, named, RuntimeException subtype -- not a bare IllegalStateException -- so a caller (MATLAB/CLI included, where it crosses the bridge the same way NoIntegerCocycleException already does) can catch and handle it specifically, and so its own message can afford to explain the situation in plain language rather than only in this engine's own internal vocabulary (top-cell ids, facet counts).