A typed scalar value produced or consumed by the reference evaluator.
Mirrors [xlog_core::ScalarType] except for floating-point types,
which are deliberately omitted: PR 2’s WCOJ supported set is
{U32, U64, Symbol} (see
super::WCOJ_SUPPORTED_KEY_TYPES); the wider non-float types
(I32, I64, Bool) are included so the evaluator can hold
non-key column values from rules where projections include them.
RefValue::Symbol carries an owned String rather than the AST’s
interned u32 — the evaluator is decoupled from
[xlog_core::symbol]’s global interner so tests stay hermetic.
Integration into a pipeline that uses interned symbols (PR 4+) is
expected to translate at the boundary.