fix(mutate): scope optimisticData and populateCache per matched key

[spokodev]

When global mutate is called with a key filter function, mutateByKey runs once
for every matched key through matchedKeys.map(mutateByKey). Two variables that
get reassigned during a mutation, optimisticData and populateCache, were declared
once in the outer internalMutate scope and shared across all those calls.

On the first matched key, a functional optimisticData is called and the shared
variable is overwritten with the resulting value. Every later matched key then
sees a non-function value and reuses the first key's optimistic data instead of
computing its own. A functional populateCache has the same problem for the
committed write. The result is that multiple cache entries show the wrong
optimistic value during a filtered mutation.

This moves both variables inside mutateByKey so each matched key gets its own
copy. Single-key mutations are unaffected. The mutator data transformer already
worked per key because it uses a fresh local variable; this brings optimisticData
and populateCache in line with that.

Added a test that seeds two keys with distinct committed values and fires one
filtered mutation with a functional optimisticData. Before the change the second
key rendered the first key's optimistic value; after it renders its own.