Declare all platform-managed columns through one mechanism, in DataJoint notation #1567

Platform-managed columns were injected three different ways. Unifying them on the DataJoint type system — and letting them be written in DataJoint notation — removes two adapter methods, fixes a live cross-backend defect (#1566), and makes a new backend inherit every platform column for free.

Implemented in #1565, for v2.3.4. The sections below are what that PR does; the original proposal is unchanged in substance.

Where things stood

_singleton was the right precedent all along; _job_* was the remaining holdout.

What the holdout cost

Verified against real servers, not inferred:

• A cross-backend defect. _job_start_time was datetime(3) on MySQL and timestamp without time zone on PostgreSQL — _job_start_time precision differs by backend: datetime(3) on MySQL, microsecond timestamp on PostgreSQL #1566. The type system already produced the right thing (timestamp(3)); the hand-written string is what lost it.
• original_type was None for all three on both backends. Cosmetic only: nothing reads it for a hidden attribute, and decoding derives json/uuid/numeric/dtype from the SQL type (heading.py:499-503). It comes free with the type-system path rather than being a reason on its own.
• Two adapter methods to keep in sync, and a third backend would have needed a third copy of each.

Grammar separated from policy

The definition language could not express a hidden attribute, for two independent reasons:

1. attribute_parser's name rule required a leading lowercase letter, so _prov = null : json raised ParseException before any check ran.
2. compile_attribute rejected a leading underscore outright.

Both were policy expressed at the wrong level. The grammar can now spell a hidden attribute; what stays forbidden is a user declaring one.

Grammar — one character class:

attribute_name = pp.Word(pp.srange("[a-z]"),  ...)     # was
attribute_name = pp.Word(pp.srange("[a-z_]"), ...)     # now

Policy — the underscore check moved out of compile_attribute, which the framework itself calls, into _reject_user_hidden_attributes, called from declare() before anything is appended. That is the single point a user's definition enters the system. No allow_reserved flag to thread through and forget, and a user declaring _foo : int still gets the same error.

One registry, in the notation the rest of the schema uses

declare.py now holds all five, and _append_platform_attributes splices them into the definition string before it is parsed:

_singleton      = 1    : bool          # singleton primary key
_job_start_time = null : datetime(3)   # when computation began
_job_duration   = null : float32       # computation duration in seconds
_job_version    = ''   : varchar(64)   # code version
_prov           = null : json          # extrinsic provenance for a row that entered from outside

Each is parsed by the normal machinery, so every platform column gets backend type mapping, a :type: comment, and original_type — with no Python construction and no adapter method. job_metadata_columns and provenance_columns are both gone. A new adapter inherits all five by implementing core_type_to_sql, which it must implement anyway.

The two retrofit paths go through the same compile_attribute call, so a column added to an existing table is indistinguishable in the catalog from a declared one:

• deploy.add_prov_column for _prov
• migrate.add_job_metadata_columns for _job_*, which also emitted backtick-quoted identifiers and so had never run on PostgreSQL at all

Constraints, and how each is met

• The user-facing ban keeps working, with the same error. tests/unit/test_declare_hidden_attribute.py pins the message and now also pins that the guard fires from a definition string rather than only from compile_attribute directly, since the check moved.
• describe() keeps omitting hidden attributes, so the describe → prepare_declare → alter round trip holds.
• Existing tables are unaffected; only new declarations change.
• tests/integration/test_migrate_job_metadata.py declares the same Computed table twice — once with the columns, once without them and then migrated — and asserts the catalog cannot tell them apart on either backend.

Sequencing

Merged into v2.3.4 rather than deferred: the parser change is one character class, and the guard it relocates is pinned by tests that moved with it. #1566 closes with it.

#1562 — a public way to read hidden attributes — remains open for 2.4. The two work the same ground from opposite sides: this one makes hidden attributes uniformly declarable, that one makes them readable.