buildmeta: remove premature Release/PackageMeta structs and
ChannelNames slice — keep only the shared vocabulary types.
uadetect: replace regex-based matching with token-based matching.
Split UA on whitespace/slash/semicolon, match lowercase tokens.
Strip xnu kernel info for Rosetta. Single Parse() entry point.
httpclient: return plain *http.Client from New(). Make Do() and Get()
free functions. Only retry idempotent methods (GET/HEAD).
Each package doc now explains what problem it solves and why it exists,
with the public interface as the only "how" detail. Implementation
notes removed from doc comments.
