[2b] Add typed select() for column selection

[davegaeddert]

What

Adds QuerySet.select(*items, flat=False, result_type=None) — typed column selection that returns honest rows (tuples, flat scalars, or dataclass instances) via a new RowQuerySet[R], never partial model instances.

User.query.where(...).select(User.id, User.email)       # RowQuerySet[tuple[int, str]]
User.query.select(User.email, flat=True)                # RowQuerySet[str]
User.query.select(User.id, User.email, result_type=UserStats)  # RowQuerySet[UserStats]

Stacked on typed-where (base of this PR), which provides the typed field-access surface.

Design

• Selectable[T] base — Field[T] and BaseExpression subclass a shared generic marker so select() can bind each column's value type. The generic base shifts the C-level solid base, so the Empty stand-ins that Field and Query cloning reassign __class__ onto also subclass Selectable to stay layout-compatible.
• Rows, not instances — reuses the values_list iterable machinery. first()/get()/iteration/slicing return rows; update()/delete() and values()/values_list() raise TypeError on a selected queryset, matching post-values() behavior.
• Fields resolve by name; expressions get internal auto-aliases via the existing values_list plumbing. result_type maps columns onto dataclass fields positionally (arity + field-name checks at select() time).

Typing

The overload ladder is keyed on Selectable[T] itself: a TYPE_CHECKING-only annotated member (__plain_selected_type__) on Selectable gives the type checker something to unify T against inside the overloaded, generic select() method. Pure-field selects get precise per-column types; a select that mixes in an expression types that expression's column as Any while the fields around it stay precise — e.g. select(D.priority, Upper("name")) → RowQuerySet[tuple[int, Any]]. This is asserted in test_select_typing.py::test_mixed_field_and_expression_row and documented in the README and code.

Not in this slice

FK traversal refs (User.profile.city) raise a clear TypeError. values()/values_list()/only()/defer()/annotate() are untouched — the consumer sweep is a later slice.

Tests / checks

./scripts/type-check plain-postgres green (ty markers accurate), full ./scripts/test green (830 passed in plain-postgres, all packages pass).