Document JSON path access in restriction and projection

[dimitri-yatsenko]

attribute.path works in both restriction and projection, and DataJoint translates it per backend. None of it was documented.

• reference/operators.md — zero mentions of JSON.
• reference/specs/type-system.md — one line: "Supports JSON path queries where available." Eleven words, no syntax, and "where available" implies a portability caveat that does not exist.
• The only pages showing the syntax were two written this week for _prov.

What this adds

A JSON path access section in the type-system spec:

• The syntax on both surfaces — Equipment & {"specs.vendor": "Acme"} and Equipment.proj(vendor="specs.vendor") — and that both emit json_value() on MySQL and jsonb_extract_path_text() on PostgreSQL.
• Nested and array paths: specs.probe.serial, specs.channels[0].
• That extracted values are text, and the path:type annotation that fixes it.
• Ordering comparisons, which need a typed projection first: proj(ch="specs.channels:unsigned") & "ch > 32".
• What is not supported: whole-object comparison raises; hidden attributes cannot be reached by the mapping form.

Two defects stated rather than left to be discovered

A reader who follows the obvious path gets wrong answers, so the section says so:

• Compare against a string. & {"specs.calibrated": True} returns no rows on MySQL and raises on PostgreSQL — the extraction yields text and the Python value is not coerced (#1564). The MySQL case is the dangerous one: an empty result reads as "none are calibrated."
• :int is not portable — works on PostgreSQL, raises on MySQL, whose RETURNING clause does not accept it. unsigned and signed work on both (#1563).

Still to come

tutorials/advanced/json-type.ipynb teaches "Filtering on JSON Content — fetch then filter in Python" and lists "Filter in Python" as an inherent property of JSON in its Design Guidelines. That is wrong — it teaches a full table scan into Python for something the database does — and it is almost certainly a workaround for #1564 mistaken for a property of the type.

Rewriting it is deliberately held until #1564 merges, so the tutorial is not rewritten twice. Tracked in #292.

Verification

Every example run against MySQL 8.0 and postgres:15. mkdocs build --strict clean; check_links.py passes across 142 pages.