WHERE expressions: wire into update/delete/show data + SQL gen (ADR-0026 steps 3-4)

Wires the stratified WHERE-expression fragment into the three
filter commands and compiles the resulting Expr to SQL.

Grammar (data.rs): the `update` / `delete` `where` clause is
now the expression fragment (`Subgrammar(&expr::OR_EXPR)`) in
place of the single `col = val` slot; `show data` gains an
optional `where <expr>` and an optional `limit <n>` (a
non-negative integer, validated at parse time). The
expression's right-hand operands are a schema-aware
`DynamicSubgrammar` so the hint panel still narrows to the
left column's type (ADR-0026 §8) — but the inner grammar is
permissive: a type-mismatched literal still parses (§7).

AST: `RowFilter::Where{column,value}` -> `RowFilter::Where(Expr)`;
`ShowData` gains `filter: Option<Expr>` and `limit: Option<u64>`.
A `RowFilter::eq` convenience constructor keeps simple-equality
call sites and tests readable.

SQL (db.rs): `compile_expr` lowers an `Expr` to a
parameterised WHERE — every literal a `?` placeholder,
identifiers `quote_ident`-quoted, `<>` for inequality. A
literal compared against a column binds through that column's
type where compatible and falls back to its syntactic shape on
a mismatch (§7 — permissive). `show data ... limit n` emits
`LIMIT ?` with an implicit primary-key `ORDER BY`, so it is a
stable "first n by primary key".

completion.rs: `invalid_ident_at_cursor` no longer mis-flags a
digit-led literal (`1`) as an unknown column now that the
WHERE operand slot also accepts a column reference; a
`ProseOnly` slot suppresses keyword candidates even when the
expected set also carries a column ident.

11 db integration tests cover AND / OR / NOT, BETWEEN, IN,
LIKE, filtered `show data`, and limit ordering; walker and
expr unit tests cover the parse surface. Type-mismatch /
`= NULL` diagnostic flagging (§7 highlight + hint) is the
remaining ADR-0026 piece.

src/app.rs

@@ -1331,7 +1331,7 @@ impl App {
             C::Insert { table, .. } => (Operation::Insert, Some(table.as_str()), None),
             C::Update { table, .. } => (Operation::Update, Some(table.as_str()), None),
             C::Delete { table, .. } => (Operation::Delete, Some(table.as_str()), None),
-            C::ShowData { name } | C::ShowTable { name } => {
+            C::ShowData { name, .. } | C::ShowTable { name } => {
                 (Operation::Query, Some(name.as_str()), None)
             }
             C::Replay { .. } => (Operation::Replay, None, None),
@@ -2502,10 +2502,10 @@ mod tests {
                 "id".to_string(),
                 crate::dsl::Value::Number("7".to_string()),
             )],
-            filter: crate::dsl::RowFilter::Where {
-                column: "name".to_string(),
-                value: crate::dsl::Value::Text("Bob".to_string()),
-            },
+            filter: crate::dsl::RowFilter::eq(
+                "name",
+                crate::dsl::Value::Text("Bob".to_string()),
+            ),
         };
         let err = crate::db::DbError::Sqlite {
             message: "UNIQUE constraint failed: Customers.id".to_string(),