feat(ffi): add vx_scalar_new_extension for extension-typed literals

[balicat]

Summary

• Tracking Issue: Tracking Issue: FFI construction of extension-typed scalars (date/timestamp scan predicates) #9689

The C API can build scan predicates against primitive, utf8, binary and
decimal columns, but not against extension columns: none of the scalar
constructors can express a value of an extension type, and the compare
kernel deliberately refuses to compare an extension column with a bare
storage-typed literal (Extension's CompareKernel requires both sides
to share the extension dtype — correctly, since e.g. timestamps in
different units must not compare raw storage).

For date and timestamp columns this leaves FFI consumers unable to push a
time window into a scan. In a .NET binding, a "series between two dates"
query over a date32 column currently has to decode each selected series'
full history and trim client-side; on a 219k-row query over an 18 MB file
we measured 521 ms for that pattern against 473 ms for an equivalent
Parquet reader pruning by row-group statistics. Everything else in the
chain already works — the engine compares extension constants efficiently
by extracting the storage scalar. The missing piece is only a way to
construct the literal through the C API.

Changes

Adds vx_scalar_new_extension(dtype, storage, err): builds an
extension-typed scalar from an extension dtype and a scalar of its
storage dtype (compared ignoring nullability; the value is copied and
neither argument is consumed). Validation is delegated to
Scalar::try_new, with explicit errors for a non-extension dtype and a
storage-dtype mismatch.

Callers obtain the extension dtype from the data source's schema or via
vx_dtype_from_arrow_schema, build the storage value with an existing
primitive constructor (e.g. vx_scalar_new_i32 for a day-precision
date), wrap it, and pass the result to vx_expression_literal in a scan
predicate.

Unit tests cover the happy path (day-precision date over i32 storage,
dtype round-trip through vx_scalar_dtype) and both rejection paths
(non-extension dtype; mismatched storage dtype). Header regenerated via
the nightly cbindgen path in build.rs.