…t.bulk_create().

This introduces support for bulk_create() on multi-table inheritance (MTI)
child models in two situations:

* Backends that can return rows from bulk inserts (e.g. PostgreSQL and
  SQLite ≥ 3.35): the top-most parent rows are inserted first and their
  primary keys are propagated through any intermediate MTI levels before
  inserting the child rows.
* Backends without RETURNING: a single MTI level is supported when each
  child instance has its parent pointer pre-set (i.e., child.pk equals
  parent.pk). In practice, bulk-create parents first, then bulk-create
  children with id=parent.pk.

Conflict handling (ignore_conflicts/update_conflicts) is intentionally not
supported for MTI in this initial version and now raises
django.db.utils.NotSupportedError.

The MTI path is factored into an internal _bulk_create_multi_table() helper;
the non-MTI behavior (including conflict options) remains unchanged. Object
state is updated so that instances returned from bulk_create() have
_state.adding = False and _state.db set appropriately.

Tests:
* Single-level and multi-level MTI with RETURNING.
* Preset-parent-pointer fast path (including UUID PKs).
* Mixed preset/non-preset on RETURNING backends.
* Respect for batch_size on the MTI path.
* Explicit parent link attname handling.
* Instance state after insert.
* Negative cases: non-RETURNING without preset pointer; conflict options on MTI.

Docs:
* Expand bulk_create() docs with a “Multi-table inheritance” subsection,
  list supported scenarios and limitations, and clarify behavior.
* Add a note in the 6.1 release notes.

Remove obsolete test asserting that MTI is universally unsupported.

…upport proxy MTI leaves with preset parent pointer.

Detect multi-table inheritance against the concrete model instead of the (possibly proxy) model so proxies don't bypass the MTI path. In the MTI code path, resolve self.model to its concrete_model to avoid inserting parent-only columns into the leaf table.

Also ensure instance state (_state.adding/db) is updated on the preset-parent fast path.

Add tests:
- bulk_create on a proxy over an MTI leaf with preset parent pointer
- bulk_create on a double-proxy over an MTI leaf with preset parent pointer

Verified locally on SQLite, PostgreSQL, and MariaDB.

…double-proxy).

Use parent_link attname and create() parents so PKs are known on non-RETURNING backends.