Move remaining expressions out of vortex-array to vortex-expr - #10768

Unbound `Expression`, binding, traversal, lambdas, proto serialization and
the expression constructors now live in `vortex-expr`; `vortex-array`
keeps arrays, the scalar function machinery, the builtins it is built on,
and `BoundExpression` with its reduce node and optimizer. The scalar
function vtable reduces over bound expressions, so keeping them next to it
lets every reduction rule stay monomorphized for expression trees as well
as array trees. `vortex-expr` re-exports `BoundExpression`. The layering is
`vortex-array -> vortex-expr -> vortex-functions`.

- `ScalarFnVTable::simplify` is generic over `ReduceNode`, like `reduce`,
  but stays a separate hook that only the expression optimizer applies: a
  simplification may rewrite a node into one whose nullability differs,
  which the array optimizer cannot accept (it must keep the exact dtype and
  defer constant masks to the kernel rules). `ReduceNode::is_same` lets
  rules such as the COALESCE rewrite compare operands.
- `new_expr`, `try_new_expr` and `try_new_bound_expr` move to
  `vortex_expr::ScalarFnExprExt`, `ArrayRef::apply`/`apply_bound` to
  `vortex_expr::ArrayExprExt`, and the expression half of the builtins to
  `vortex_expr::ExprBuiltins`. `ExprDisplay` moves to
  `vortex_array::scalar_fn::display`, and the tree renderer to
  `vortex_array::expr::display`.
- `ArrayRef::take_slot_unchecked`/`put_slot_unchecked` become public, like
  `with_slots`, so row-count substitution in `vortex-expr` still rewrites
  lazy arrays in place.

`vortex_array::expr` otherwise keeps only `stats`, which moves with the
aggregate split. The core builtins' expression-level tests move to
`vortex-expr`; the few array-level tests that used expressions now go through
`ArrayBuiltins`. The `expr_*` benches move to `vortex-expr`, and their
CodSpeed counts and durations move with them.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Ps4mp7KF5c4p66aoSoXx5j
Signed-off-by: Claude <[email protected]>

`select` and `is_null` are not needed by vortex-array's own arrays, so they
move to vortex-functions with their constructors, builder methods
(`ArrayFunctions::is_null`, `ExprFunctions::is_null`), tests and pruning
rewrites (`vortex_functions::pruning`). `not` stays in vortex-array, because
validity inversion and the `sum_v2` aggregate negate arrays lazily.

- `get_item` read through `select` by naming it. It now asks its child through
  the new `ScalarFnVTable::forwarded_field` hook, which reports the child whose
  field an output field passes through unchanged. `select` implements it, so
  `get_item(select(x))` still reduces to `get_item(x)` for arrays and
  expressions.
- `IsNull` and `IsNotNull` share the and/or null-check reduction through the
  public `reduce_null_check`; each passes its own nested check.
- The field-access analyses `referenced_field_paths` and
  `make_bound_free_field_annotator`, which look through `select`, move from
  vortex-expr to `vortex_layout::analysis`, next to the scans that use them.

Signed-off-by: Claude <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Ps4mp7KF5c4p66aoSoXx5j

…ex_array::stats

`Stat`, `Precision`, the stat bounds and the `StatsProvider` traits describe the
statistics every array carries, not expressions, so they join the per-array
statistics cache (`ArrayStats`, `StatsSet`) in `vortex_array::stats`.
`vortex_array::expr` now holds only `BoundExpression` and its optimizer.

Callers move from `vortex_array::expr::stats` to `vortex_array::stats`. The
umbrella crate's `vortex::expr::stats` keeps re-exporting the statistics
vocabulary alongside vortex-expr's stat expressions.

Signed-off-by: Claude <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Ps4mp7KF5c4p66aoSoXx5j