feat: documents and session tokens

context/README.md

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+# JSON-LD context
+
+`scienting-1.jsonld` is the context that, together with the W3C Web Annotation context, makes every annotation layer valid JSON-LD.
+It is published at `https://scient.ing/ns/scienting/1.jsonld`, and every layer and annotation lists both:
+
+```json
+{"@context": ["http://www.w3.org/ns/anno.jsonld", "https://scient.ing/ns/scienting/1.jsonld"]}
+```
+
+The context maps the terms the formats add to annotations (`kind`, `status`, `audience`, `receipt`, `verdict`, `AnchorSelector`, `anchor` and others) to IRIs under `https://scient.ing/ns/scienting#`.
+`anchor` and `exercise` are JSON literals, so a JSON-LD processor keeps a `scienting-anchor/1` record intact instead of expanding it.
+
+A Web Annotation tool that knows nothing of this context still reads the annotation's body and its `TextQuoteSelector`; the context matters only to tools that process the data as linked data.
+[Chapter 18](../spec/18-layers.md) defines layers and annotations.