Function dataTableToChunkSource

Convert a legacy DataTable into a ChunkSource by repacking its columnar data into the canonical per-layer interleaved layout.

Detects SH band count from the highest f_rest_* index, identifies non-standard columns as other-layer extras, and copies each gaussian's fields into the appropriate per-layer buffer.

Used during the 3.0 migration by readers that haven't yet been ported to native chunked decoding — they call this at the end of their existing decode to upgrade to the new return type.

When indices is supplied, only those rows are repacked, in that order — a direct ordered-subset gather (e.g. the LOD writer's per-unit gather), avoiding a separate DataTable.clone({ rows }) copy.