test: guard third-party Solid client interop in CI — @inrupt/solid-client drives Cistern 0.2.0 8/8 today

[josepharuja]

Measured result

@inrupt/solid-client@1.23.1 — the most widely used Solid client library — drives Cistern 0.2.0 with 8/8 passing and zero Cistern-specific code:

Operation	Result
read root container as a dataset	PASS
create container (saveSolidDatasetAt)	PASS
write an RDF document (Thing + foaf:name)	PASS
read back, exact round-trip of the value	PASS
containment — parent lists the child	PASS
binary upload (saveFileInContainer)	PASS
binary read back (getFile)	PASS
unauthenticated read refused	PASS (401)

Auth was a custom fetch adding Authorization: Bearer <owner token>; Cistern has no Solid-OIDC yet, so there is no DPoP path to exercise. Run against ghcr.io/enrichmeai/cistern:0.2.0, both containers on one Docker network so minted IRIs matched.

Why this matters

1. It is conformance evidence of a kind the CTH currently cannot give. cth/BASELINE.md reads 0/41 because the harness never reaches an assertion. A third-party client exercising the real protocol surface is the strongest external signal available today.
2. It reduces the urgency of the TypeScript half of T7.9 Client SDKs — thin Java and TypeScript clients for applications #101. If the standard client already works, the argument for shipping our own JS client — a second toolchain in a JVM-native project, competing with Inrupt's — is much weaker. The Java client may still earn its place; the TS one needs re-justifying.
3. It is a regression risk that nothing currently guards. Any change to content negotiation, containment serialisation, ETags or status codes could silently break the ecosystem's default client.

Scope

• Adopt the probe as a CI job: start the built image, run the client against it, assert all operations.
• Pin the client version; treat a failure as a conformance bug in Cistern, not in the test.
• Document in docs/INTEGRATION.md that the standard client works, with the bearer-fetch snippet — this is a better answer to "is there an SDK?" than an SDK.

Known trap to document

@inrupt/solid-client → jsonld@5.2.0 → @digitalbazaar/http-client → esm@3.2.25, which hard-crashes Node 23 (native assertion in InternalModuleStat). Works on Node 20. A developer trying the obvious thing on current Node hits this before reaching Cistern — name the supported Node version.

DoD

• CI job runs the interop suite against the built image on every PR.
• INTEGRATION.md documents the standard-client path and the Node version constraint.
• T7.9 Client SDKs — thin Java and TypeScript clients for applications #101 updated with a decision on whether the TS client is still wanted.