State publicly where specimen / sample-type terms belong (organoid, cell culture, PBMC, biopsy)

Problem

Single-cell schemas need to describe what kind of material was sampled, not just where it came from. Uberon has essentially none of this vocabulary:

Because nobody owns the join, every schema improvises its own escape hatch. CELLxGENE and scFAIR both carry a tissue_type enum alongside the Uberon term:

scFAIR 7.1.0 is explicit about the consequence: when tissue_type is cell line the tissue_ontology_term_id field takes a Cellosaurus ID, and when it is primary cell culture it takes a CL ID. One field holds three different kinds of entity, disambiguated by a sibling field.

Proposal

Much of this legitimately belongs to OBI (specimen and sampling process) and Cellosaurus (cell lines), not to Uberon. That is a defensible position — but it is not written down anywhere, so downstream schemas keep reinventing the workaround.

Publish a short statement in the Uberon docs saying where each of these belongs, and coordinate with OBI on anything genuinely missing. Deciding and documenting the boundary is worth more here than minting terms.

Why this is actionable now

This is a scope/ownership statement, not a modelling change. It needs no new relations.

Context

Single-cell coverage gap report §6, modernization strategy §3.2.

Possibly related: #2898 (data element harmonization).

Read §6 of docs/single-cell/gap-report.md and §3.2 of docs/single-cell/modernization-strategy.md, then review the examples in the issue. Document where organoid, culture, biopsy, blood, and related terms belong across Uberon, OBI, and Cellosaurus, and identify anything needing OBI coordination.