Reject in-memory sorting on complex fields

[woksin]

Fixed

• In-memory sorting on a complex field (object, array, map, or a concept over one) now answers 400 malformedRequest, or a validation-result frame once an observable stream has started, instead of returning an unreliable order. (Sorting on complex fields is silently ignored #174)

Deprecated

• Generated sortBy helpers for record, nested-model, array, map, and polymorphic fields are deprecated and will be removed in the next major release; sort on a scalar field instead. (Sorting on complex fields is silently ignored #174)

[woksin]

Reviewer notes: this is the third design for #174. The first two (branch fix/174-complex-sorting, kept for reference) changed the MongoDB/Drizzle providers, paged results, error shapes and the order of client output and interception. Full reviews found regressions there, including a critical bypass of read-model interception. This version only adds a scalar check to Arc's existing in-memory sort rejection, and only marks complex-field sortBy helpers @deprecated.

Compared with main (cbb421a), the generated proxies are identical for the generator fixtures (9 files) and the Tasks/Library samples (12 files). The only differences are 8 deprecation comments in the ProxyComparison snapshot. So nothing in the generated public API is removed or renamed, and patch applies.

Follow-ups: #176 (raw concept/Date ordering), #177 (remove the deprecated helpers in the next major), #178 (type indirect and generic concept subclasses as their scalar).

Local: every root ci step passed (3,371 specs), plus yarn test:conformance (298 HTTP checks and the live proxy comparison). Full and quick cross-provider reviews ran; all confirmed findings are fixed.