RFC: Portolan as a pure STAC profile (v0.2.0) — thin index, derived capabilities, no Portolan extension

[jatorre]

RFC / discussion PR — not for merge as-is

Proposal to converge Portolan on "STAC as a thin index" after the 2026-07-09 workshop.

This PR adds only a proposal — it does not rewrite any canonical spec/ file:

• context/shared/adr/0057-portolan-pure-profile.md — the decision record (Status: Proposed)
• spec/rfc/pure-profile-v0.2.0/ — the full RFC, draft conformance.md + capabilities.md, and a
  validated, STAC-Browser-rendered reference example set (buildings single-table + cadastre
  multi-table model with OSI semantics)

The gist

• No Portolan STAC extension — every field is derivable or already homed elsewhere.
• Identity + version via conformsTo; capabilities derived, not declared (contract per capability).
• Drop portolan:geospatial; bbox is uniformly an AOI.
• AGENTS.md + README.md replace llms.txt (supersedes ADR-0052).
• Multi-table models = one collection, tables as assets; relations in OSI (incl. spatial-predicate).
• Visualization = capability + contract, not a mandated format (the PMTiles debate).

Evidence

stac-node-validator v2.0.0-rc.3 → 3/3 pass (all extensions incl. proposed osi:relations);
renders in STAC Browser v5.0.0-rc.1.

Asks

1. React to the direction (ADR-0057) — merge as one or split into 5 focused PRs (see RFC).
2. The PMTiles reframe (capability + contract).
3. Four core judgment calls (schema MUST/SHOULD · versions.json core vs capability · format allowlist · per-collection docs).

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