command: Operand carries a source span

Each WHERE-expression Operand now records the byte span of the
terminal it was built from — the precise per-literal highlight
target for an expression WARNING (finishing ADR-0027 §2's
highlight/hint wiring). parse_operand captures MatchedItem::span;
the RowFilter::eq convenience constructor uses Operand::NO_SPAN.

PartialEq is hand-written to ignore the span — it is editor
metadata, so Command equality stays whitespace- and
position-independent, which the Expr test corpus relies on.
No behaviour change; 1100 tests still pass, clippy clean.

src/dsl/walker/mod.rs

@@ -542,7 +542,13 @@ fn predicate_warnings(
 }
 
 const fn is_null_literal(operand: &Operand) -> bool {
-    matches!(operand, Operand::Literal(crate::dsl::value::Value::Null))
+    matches!(
+        operand,
+        Operand::Literal {
+            value: crate::dsl::value::Value::Null,
+            ..
+        }
+    )
 }
 
 /// If one operand is a known column and the other a non-null
@@ -556,8 +562,10 @@ fn pair_type_mismatch(
     columns: &[crate::completion::TableColumn],
 ) -> Option<String> {
     let (column, literal) = match (a, b) {
-        (Operand::Column(c), Operand::Literal(v))
-        | (Operand::Literal(v), Operand::Column(c)) => (c, v),
+        (Operand::Column { name: c, .. }, Operand::Literal { value: v, .. })
+        | (Operand::Literal { value: v, .. }, Operand::Column { name: c, .. }) => {
+            (c, v)
+        }
         _ => return None,
     };
     // `null` fits any column; `= NULL` is flagged separately.