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

sql/store.sql

@@ -68,7 +68,7 @@ CREATE TABLE layers (
 CREATE TABLE annotations (
   id VARCHAR(256) NOT NULL,
   layer VARCHAR(256) NOT NULL,
-  body_kind VARCHAR(24) NOT NULL CHECK (body_kind IN ('gloss', 'claim-link', 'symbol', 'comment', 'caveat', 'question', 'exercise', 'citation-context', 'prerequisite', 'crosslink', 'translation', 'description', 'generated')),
+  body_kind VARCHAR(24) NOT NULL CHECK (body_kind IN ('gloss', 'claim-link', 'symbol', 'comment', 'caveat', 'question', 'exercise', 'citation-context', 'prerequisite', 'crosslink', 'trace', 'translation', 'description', 'generated')),
   category VARCHAR(12) CHECK (category IN ('comment', 'context', 'critique', 'question', 'correction', 'feedback', 'resource')),
   status VARCHAR(10) NOT NULL CHECK (status IN ('proposed', 'confirmed', 'rejected', 'withdrawn')),
   receipt VARCHAR(32),