Add complete DELETE and UPDATE support to MemTable as a reference
implementation for the TableProvider DML hooks.

Implementation:
- `delete_from()`: Evaluate filter predicates, remove matching rows
- `update()`: Evaluate filters, apply SET expressions to matching rows
- `DmlResultExec`: ExecutionPlan returning affected row count
- Correct SQL NULL semantics (NULL predicates preserve rows)

Tests (812 lines):
- DELETE: basic, all rows, predicates (=, >, AND, OR, LIKE, BETWEEN,
  IN, NOT IN, IS NULL, IS NOT NULL), NULL handling, functions, types
- UPDATE: basic, SET expressions, multi-column, CASE, functions,
  column references, NULL handling, error cases

All sqllogictest tests pass:
  cargo test --test sqllogictests -- dml_delete dml_update