ChunkedLayoutDecoder throws ClassCastException on chunked List-dtype columns

[dfa1]

Summary

ChunkedLayoutDecoder.decodeChunkedLayout crashes with a raw ClassCastException (not VortexException) when a chunked-layout column with more than one flat child has a DType.List dtype. Violates the security contract in CLAUDE.md: every malformed/unsupported input must throw VortexException, never a raw JDK exception.

Root cause

ChunkedLayoutDecoder.java:129-134 special-cases Bool, Utf8/Binary, then falls through to an unconditional (DType.Primitive) dtype cast for everything else:

} else {
    PType ptype = ((DType.Primitive) dtype).ptype();
    data = switch (ptype) { ... };
}

A List-typed column (or Struct) with >1 flat chunk hits this branch and the cast throws ClassCastException instead of a handled VortexException.

Repro

Discovered via the Raincloud conformance corpus (#205): both squad-v2 (list answers.answer_start) and oasst1 (nested list columns) trigger it.

java.lang.ClassCastException: class io.github.dfa1.vortex.core.model.DType$List cannot be cast to class io.github.dfa1.vortex.core.model.DType$Primitive
	at io.github.dfa1.vortex.reader.layout.ChunkedLayoutDecoder.decodeChunkedLayout(ChunkedLayoutDecoder.java:134)
	at io.github.dfa1.vortex.reader.layout.ChunkedLayoutDecoder.decode(ChunkedLayoutDecoder.java:41)

Expected

Either a genuine chunked-List decode path, or (at minimum) a VortexException with a clear "unsupported dtype for chunked layout" message instead of the raw cast failure.