Add a reusable promise cache for asynchronous computations #2229

Several proposed performance changes need the same asynchronous caching
pattern: return an existing promise for a key, or create and cache one on a
miss. Caching the promise deduplicates concurrent calls as well as later calls.

A shared utility would avoid repeating this logic and provide consistent
choices for:

• Map or WeakMap storage, depending on key lifetime;
• eviction after rejection, so transient failures can be retried;
• per-instance method caching through a decorator; and
• custom key derivation for methods with multiple arguments.

The utility itself does not alter an existing hot path. Its performance effect
should be measured in the changes that adopt it.