perf(viewer): reuse click handlers instead of rebuilding them per render - #556

Six factories built a fresh closure for every dropdown option on every
render and registered it in globals(). Mesop memoizes handler ids on the
function object itself (compute_fn_id is an unbounded lru_cache), so a new
closure per render meant a permanent entry per render: 300 renders of a
40-option dropdown added 12,000 entries instead of 40.

Hoist the factories to module level and memoize them by value, so the same
option yields the same function object and mesop's table stays flat. The
globals() writes go with them; nothing ever read those names.

Handler name is load-bearing and stays: mesop derives identity from
name plus source, and all handlers from one factory share source, so
without it every option in a dropdown would dispatch to the same value.
Names now carry a short digest of the raw value, since the old sanitizer
mapped "a.b" and "a-b" to one name and silently merged them.

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