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.

tests/iteration5_export_import.rs

@@ -357,7 +357,7 @@ fn end_to_end_export_then_import_real_project() {
 
     // Round-trip: the inserted row is back.
     let data_view = rt()
-        .block_on(async { imported_db.query_data("Customers".to_string(), None).await })
+        .block_on(async { imported_db.query_data("Customers".to_string(), None, None, None).await })
         .expect("query data");
     assert_eq!(data_view.rows.len(), 1);
     // Serial id auto-filled to 1; Name was the inserted value.