feat: trace annotation body for traced citations; paperspec.traces

A trace follows one citation of a sentence of a citing paper to a passage of the
cited paper (spec/18 section 18.5). Minor change: a new body kind, optional
properties and two rules, with no valid file made invalid.

- annotation schema branch (source, selector, reference, note, verdict, proposer;
  proposer required for a Software creator); body_kind in sql/store.sql; the note
  and proposer terms of the JSON-LD context; rules layer.trace.source and
  layer.trace.target.
- paperspec.traces: resolve(annotation, citing_state, cited_state, same_as) gives
  current, changed, stale, unlinked or misresolved with open and cites; problems,
  disputes, disputed.
- fixtures/records/traces/ on writer2027fixed-ynr7.v3 and vectors/traces.jsonl
  (29 cases); check.py runs the record checks over both layers; counts, glossary,
  examples and CHANGELOG updated.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uy4ZYbWRoCdUtWLEfigysY

docs/concepts.md

@@ -376,8 +376,10 @@ Claims are not defined in this repository.
 A comment, caveat or question also says what it is about with a `category` from a fixed list: a general `comment`, `context`, a `critique`, a `question`, a `correction` of an error in the paper, `feedback` for the authors, or a `resource`.
 Model-written text is always a `generated` annotation, never source, and a model's annotation stays `proposed` until a person confirms it.
 Whether a generic Web Annotation client displays an `AnchorSelector` has not been tested; the fallback exists so that it can still use the quote.
+A `trace` annotation follows a citation to the passage it rests on: it targets the citing sentence and its body names the reference, the cited paper and the passage in it.
+Whether a trace still leads there is computed from both papers rather than stored: `current`, `changed`, `stale`, `unlinked` or `misresolved`.
 Layers are `candidate`.
-Pinned by the layer records in `fixtures/records/layer/`.
+Pinned by the layer records in `fixtures/records/layer/` and `fixtures/records/traces/`, and by [vectors/traces.jsonl](../vectors/traces.jsonl).
 See [chapter 18](../spec/18-layers.md).
 
 ## Exercises