Phase 2: record/metadata layer — a Python DataProvider (CRUD+query) mirror of the zodal contract #80

Follow-up to #78 / #79 (which shipped the content half as dol.content). This issue tracks the record/metadata half of the content–metadata bifurcation: a queryable facade for the small rows that sit next to the blobs.

Motivation

dol.content gives a flat blob store + ContentRef. The other half of the pattern is a collection/record facade with CRUD + query, so an app can list/filter/paginate records whose large fields are offloaded to a content store (holding a ContentRef in their place). Our TypeScript side (zodal) already models exactly this as DataProvider<T>, and a Python mirror keeps the two ecosystems contract-compatible.

Proposed scope

Mirror zodal's @zodal/store contract in Python:

• DataProvider-style facade — get_list(params) -> {data, total}, get_one(id), create, update, update_many, delete, delete_many (+ optional upsert). Structured sort/filter/search/pagination params, with a shared client-side filter_to_function fallback for backends that can't push predicates down.
• Capability discovery — a ProviderCapabilities-style object so callers degrade gracefully (which ops / which pushdowns a backend supports).
• Bifurcated composition — a bifurcated_provider(metadata_provider, content_provider, content_fields) that splits records on write (blobs → dol.content, rest → the metadata store), replaces content fields with ContentRefs on read, and runs the metadata-is-commit-point saga (delete content-first; compensate metadata on content-write failure).
• wrap_provider codec layer (the analog of wrap_kvs; zodal's wrapProvider already cites dol).

Placement (per the zero-dep rule)

• The abstract contract + in-memory/dict reference provider + bifurcated composition are pure stdlib → candidate for dol (or xdol if any helper needs a dep).
• Concrete queryable backends (SQL, Supabase, etc.) carry deps → their own packages, never core dol.

Reference

• zodal contract: zodal/packages/store/src/{data-provider,capabilities,bifurcated-provider,wrap-provider}.ts
• design corpus: zodal/docs/research/ + misc/docs/dol_content_metadata_bifurcation.md

Deferred from the phase-1 decision ("blob now, DataProvider later").