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/comparison.md

@@ -428,7 +428,7 @@ Keep using it as a reading and distribution format.
 *Status.*
 The specification is version 0.1.0.
 `registry/formats.json` lists 32 formats: 19 are `candidate` (complete, with vectors and fixtures, ready to implement) and 13 are `provisional` (complete; expected to change as implementations gain experience): project, bundle, dataset, digitization, view, snapshot, draft-ids, exercise, submission, grade, checklist, log-entry and checkpoint ([spec/00 §0.4](../spec/00-introduction.md#04-status)).
-The repository holds 24 chapters, 43 JSON Schemas, 25 vector files, five valid fixture packages, one of them also zipped, and 31 invalid ones, each naming the rule it breaks, and a reference checker, `paperspec check`.
+The repository holds 24 chapters, 43 JSON Schemas, 26 vector files, five valid fixture packages, one of them also zipped, and 31 invalid ones, each naming the rule it breaks, and a reference checker, `paperspec check`.
 Licenses: prose CC BY 4.0; schemas, SQL, the JSON-LD context, registries, vectors and fixtures CC0 1.0; the code in `src/` Apache-2.0 ([README.md](../README.md#licenses)).
 
 *No adoption claim.*