v0.17.0

Added

• Product, Offset and Copied: the commonest arithmetic, said as data.
  They compute nothing Derived could not. They exist because of what Derived
  costs: it takes a callable, a callable cannot be honestly digested, and a
  shape holding one is refused by template_database. So a column as ordinary
  as total = quantity * unit_price took a whole declaration out of the reuse
  that turns a forty-second build into a hundred-millisecond clone -- and
  build() kept working, so nothing said what it had cost.

  The refusal stays; it is right. These three are pure data, implement
  Canonical, and hash. Derived remains the answer for computation that
  really is code.

  Offset is also the same-row half of After, which is parent-scoped only.
  Its gap is fixed rather than spread across a window, because a due date thirty
  days after an issue date is a term and not a distribution.

Changed

• A scaled world can now be built over a session world. The two pytest
  surfaces this package ships wanted the same tables in any application with one
  model graph, and were mutually exclusive: the session world holds its rows for
  the whole run, so the scaled build met a table that was not empty and refused.
  The documented answer -- give the two different models -- is not available to a
  project whose plan assertions and growth assertions are about the same flow,
  because that is what the application is.

  It was also order-dependent, which is what made it worth fixing rather than
  documenting better: a suite whose growth tests happened to be collected first
  passed, and the same tests named in the other order failed.

  scaled_world now empties the tables its shape declares before building.
  Nothing is snapshotted, and nothing needs to be: this happens inside the
  atomic block it already rolls back, so whatever was there comes back when the
  block ends. A table whose keys are Disjoint is left alone, mirroring the
  exemption build already makes, so the documented hybrid -- parents made by
  your code, children made here -- keeps working.

  build() keeps its refusal. It has no transaction of its own to undo and
  would be destroying rows for good.

  The two statement-count constants moved: one TRUNCATE on PostgreSQL, one
  DELETE per declared table elsewhere. Both are properties of the declaration
  rather than the factor, so what those tests pin -- fixed on PostgreSQL at
  every factor, a curve off it -- is unchanged.

Fixed

• After across a parent column of a different kind is refused.
  date + timedelta is a date, so a DateTimeField child whose parent column
  was a DateField was filled with dates: COPY accepted them and they landed
  as naive midnights. A world whose orders went on sale in 2024 had shows
  starting in 1900, and the only signal was Django's own per-row
  RuntimeWarning in the middle of a build that prints thousands of lines.

  Both field types are on _meta and both are known before a row is generated,
  so this belonged in the class of things already refused at declaration time.
  Only After is checked, deliberately: After is parent + offset written
  into this column, so the two describe the same quantity by construction, while
  a Derived reading a parent column may legitimately convert -- turning a
  timestamp into a count of days is an ordinary thing to declare.