Register explicit casts between agtype and jsonb, enabling:

  SELECT properties(n)::jsonb FROM cypher(...) AS (n agtype);
  SELECT '{"key": "value"}'::jsonb::agtype;

  -- Use jsonb operators on graph data:
  SELECT (props::jsonb)->>'name' FROM cypher(...) AS (props agtype);

Implementation uses SQL language functions that go through proven
text-intermediate paths:
  agtype -> jsonb:  agtype_to_json() -> json::jsonb
  jsonb -> agtype:  jsonb::text -> text::agtype

This approach is safe because agtype extends jsonb's binary format
with types (AGTV_INTEGER, AGTV_FLOAT, AGTV_VERTEX, AGTV_EDGE,
AGTV_PATH) that jsonb does not recognize, making direct binary
conversion unreliable.  The text roundtrip handles all value types
correctly including graph types (vertex/edge properties are
extracted as JSON objects).

All 31 regression tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…asts

- Fix comment in agtype_coercions.sql: document json-intermediate path
  (agtype_to_json -> json::jsonb) instead of incorrect "text intermediate"
- Add agtype_jsonb_cast regression test covering: string/null/array/object
  agtype->jsonb, all jsonb scalar types->agtype, roundtrips, vertex/edge
  ->jsonb with structural key checks, NULL handling
- Register agtype_jsonb_cast in Makefile REGRESS list

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>