Reduce all contributions for ONE command key into a ReducedDefinition.
The heart of "reduce up the chain": the registry calls this with the contributions
gathered along the active scope chain, ordered least → most specific.
Cross-scope reduction lives here; within-scope merging (successive updateCommand calls
on one provider) happens before this, so each defs entry is one scope's merged value.
Reduce all contributions for ONE command key into a ReducedDefinition. The heart of "reduce up the chain": the registry calls this with the contributions gathered along the active scope chain, ordered least → most specific.
Cross-scope reduction lives here; within-scope merging (successive
updateCommandcalls on one provider) happens before this, so eachdefsentry is one scope's merged value.