clean code

[matt-beanland]

Backlog — Non-idiomatic Ash patterns
High
Silent error swallowing in reduce_while / {:halt, []}
Three files share the same anti-pattern: on failure they halt with an empty list [], discard the original error, and surface a generic string. The real error is gone.

lib/diffo/provider/components/instance/extension/relationship.ex — relate_instance/2
lib/diffo/provider/components/instance/extension/party.ex — party relating loop
lib/diffo/provider/components/instance/extension/place.ex — place relating loop
Ash way: propagate the actual error tuple; let Splode accumulate it.

Medium
Unreachable / dead code in IsRelatedDifferent
lib/diffo/validations/is_related_different.ex lines 29–31 — after the :error -> branch returns :ok, two further expressions ({:ok, nil} and a second :ok) are unreachable dead code left over from editing. Confusing to read, should be cleaned up.

Low
reduce_while for sequential side effects in build helpers
characteristic.ex and feature.ex use nested Enum.reduce_while for what is essentially a for-each-with-early-exit. Functional but hard to follow. Could be replaced with Enum.reduce_while → with chains or explicit recursion where error propagation is the real intent.

lib/diffo/provider/components/instance/extension/characteristic.ex
lib/diffo/provider/components/instance/extension/feature.ex
Biggest bang for buck is the three silent-halt files — they're the same mistake repeated and they destroy error information.

[matt-beanland]

High — silent error swallowing (relationship.ex, party.ex, place.ex)
All three relate_instance/2 functions replaced {:halt, []} + generic error string with proper {:halt, {:error, error}}. The relationship one also simplified by collapsing the nested case direction / case Provider.create_relationship into an attrs build step followed by a single create_relationship call. The result is the actual upstream error propagates to the caller instead of vanishing.

Medium — dead code (is_related_different.ex)
The three-expression :error -> arm reduced to the single :ok that was always returned.

Low — mixed accumulator types (characteristic.ex, feature.ex)
All reduce_while accumulators changed from bare [] to {:ok, []}, matching the halt type. The callers (set_characteristics_argument, set_features_argument) updated to match {:ok, []} / {:ok, list} / {:error, error} — no more mixed list/error type from the same expression.