Add a locale-keyed cache for no-argument translation lookups so repeated
literal keys resolve without redoing the lookup chain. Reset on locale change
and on locale override application.
Ported from phase3-measured-perf (cherry-pick -x 970dd88), adapted to current
main (hoisted the per-locale cache-table creation; excludes the branch's audit
doc and gitignored test).
_l10n:translate now short-circuits the common no-argument case
(select('#', ...) == 0), returning the translation directly and skipping
the {...} allocation, the tostring loop, and safeFormat. Behavior matches
the slow path: a successful no-arg lookup already returned the raw value,
and missing/invalid entries fall back to the key. Cherry-picked from
phase2-lua50-sweep.
Xurkon