feat: add pluggable slice merge strategies

[donejeh]

Merge overwrites slices wholesale, so anyone who wants a later config layer to add
to a list rather than replace it has to write a WithMergeFunc from scratch and
reimplement the deep merge around it. This adds three ready-made strategies to the
maps package and exposes each one as a Load option.

• MergeAppendSlices concatenates the source slice after the destination one.
• MergeByIndex pairs elements at the same index, deep-merging maps and recursing
  into nested slices.
• MergeByKey(field) matches map elements on a named field, deep-merging the ones
  that match and appending the ones that do not. When a field value repeats on the
  destination side, the first element wins. An empty field name returns an error.

All three keep the existing behaviour for nested maps and non-slice values, and they
handle concrete slice types such as []string rather than only []any. Values taken
from the source are deep-copied, so mutating the source after a merge cannot reach
into the merged result.

WithMergeAppendSlices(), WithMergeByIndex() and WithMergeByKey(field) wire these
into Load. Tests cover both the maps functions directly and the Load options
end to end.

One note on module ordering: the Load options live in the root module but call into
the maps module, which the workspace resolves locally. Releasing the root module
would need a maps tag first. Happy to split this into a maps-only PR and a
follow-up for the Load options if you'd prefer that sequencing.