ui : move discover download types, enums and constants to lib
Extract the download-options shared symbols out of the component-local
download-options.utils.ts into their lib-scoped folders: QuantOption,
DownloadEntryState, BitDepthRow and SelectableFile into $lib/types; a new
SelectableFileKind enum (replacing the 'main'/'draft'/'aux' string union)
into $lib/enums; SPEC_TYPE, the bit-depth buckets and the draft label into
a new models-discover-download.constants.ts. The util keeps only its
dedicated functions, with named constants for its magic values.
Assisted-by: llama-ui:Qwen3.8-Flash-Next
