Cached parsed data types.

[rustyeddy]

yes, that is a good point, no need to constantly re-parse strings like this. At some point it may be good to create a Rate cache per normalized string.

Originally posted by @rustyeddy in #84

[rustyeddy]

Narrow fix (the zero-risk half of this issue — hoisting bar.go's own "0.5" constant parse to package init) on feature/85-cache-half-rate — PR: #97. The broader general Rate/string-parsing cache stays deferred per the original scoping note, pending actual performance evidence.