Added subqueries

• Operator: add EXISTS_SUBQUERY and NOT_EXISTS_SUBQUERY enum values
• Condition: guard matches() against Query-valued values; extract matchesEq()
  helper to keep cyclomatic complexity within the 10-method limit
• Query: add fromSubquery, fromAlias, joins (List), and
  selectSubqueries (List) fields with accessors
• JoinClause: new immutable holder for plain-table and derived-table JOINs;
  inner enum Type {INNER, LEFT, RIGHT, CROSS}
• ScalarSelectItem: new immutable holder for (SELECT ...) AS alias items in
  the SELECT clause
• AbstractSqlDialect: rewrite render() and renderDelete() to support all new
  query fields; add appendFromClause, appendJoinClauses, appendSelectColumns
  (with scalar-subquery param collection), appendSubqueryCondition and five
  private helpers; deterministic param ordering: scalar SELECT -> FROM ->
  JOIN -> WHERE

QueryBuilder (SELECT):

• whereInSubquery(col, Query): WHERE col IN (SELECT ...)
• whereEqualsSubquery(col, Query): WHERE col = (SELECT ...)
• whereExistsSubquery(Query): WHERE EXISTS (SELECT ...)
• whereNotExistsSubquery(Query): WHERE NOT EXISTS (SELECT ...)
• fromSubquery(Query, alias): FROM (SELECT ...) AS alias
• joinSubquery(Query, alias, on): INNER JOIN (SELECT ...) AS alias ON ...
• selectSubquery(Query, alias): (SELECT ...) AS alias in SELECT clause
• build() propagates all four new Query fields (fromSubquery, fromAlias,
  joins, selectSubqueries)

DeleteBuilder (DELETE):

• Remove duplicate rendering logic (SIMPLE_OPS map, handle*Operator methods)
  and delegate to AbstractSqlDialect.renderDelete() via new toQuery() helper
• toQuery() sets limit = -1 to suppress spurious LIMIT 0 on MySQL / SQLite
• whereInSubquery(col, Query): WHERE col IN (SELECT ...)
• whereExistsSubquery(Query): WHERE EXISTS (SELECT ...)

Unit tests (54 new assertions across 7 files):

• SubqueryConditionTest: WHERE IN subquery across STANDARD/MYSQL/SQLITE,
  WHERE NOT IN subquery, WHERE col = (SELECT ...)
• ExistsSubqueryTest: EXISTS, NOT EXISTS, MySQL quoting, multi-condition params
• FromSubqueryTest: FROM (SELECT ...) AS alias across all three dialects,
  outer WHERE combined with FROM subquery
• JoinSubqueryTest: INNER JOIN subquery STANDARD/MYSQL, param ordering
• ScalarSelectSubqueryTest: scalar SELECT STANDARD/MYSQL, subquery-only SELECT
• NestedSubqueryParamOrderTest: deterministic param order (scalar->FROM->JOIN->WHERE),
  3-level nesting
• DeleteSubqueryIntegrationTest: DELETE + IN subquery STANDARD/MYSQL/SQLITE,
  multi-condition param ordering, DELETE + EXISTS subquery

Execution matrix (SqlExecutionMatrixTest — 54 parameterized tests):

• Spins up an in-memory SQLite database via sqlite-jdbc
• Executes every SELECT operator (all 14 + subquery variants), INSERT,
  UPDATE, DELETE, and CREATE TABLE combination against a live JDBC connection
• DML cases run inside savepoints that are always rolled back
• pom.xml: add sqlite-jdbc:3.46.1.3 and junit-jupiter-params:6.0.3
  as test-scoped dependencies

Updated README

• Features: add subquery bullet
• Quick Start: new Subqueries section with six annotated examples
  (WHERE IN, EXISTS/NOT EXISTS, scalar EQ, FROM derived-table,
  JOIN subquery, scalar SELECT subquery)
• All Operators table: add whereInSubquery, whereEqualsSubquery,
  whereExistsSubquery, whereNotExistsSubquery rows; fix whereNotEquals
  and whereLessThan to correctly show DeleteBuilder-only availability
• QueryBuilder Builder Reference: remove non-existent whereLessThan /
  whereNotEquals, add all seven new subquery helpers with comments
• DeleteBuilder reference: add whereInSubquery and whereExistsSubquery