Mount a new instance. Returning undefined is the provider's way
of declining (e.g. preconditions not met — no View available,
renderer inspector not yet exposed). Callers see undefined
back from open(id).
Locate an already-mounted instance, if any. Returning undefined
makes the registry fall through to create.
OptionalonOptional side-effects when revealing an existing instance —
focus a specific model, re-bind a target, etc. The registry calls
this after find succeeds and before returning the panel.
Adapter that teaches the registry how to find / construct one particular panel or tool. Every concrete panel in the built-in set registers one of these.