Feature: Engine-agnostic SQL generation and live schema inspection (SQL Generator & Inspector)

[JJJ]

Goal

Introduce a decoupled SQL generation and schema introspection layer inside BerlinDB to support database engine agnosticism (MySQL, SQLite, PostgreSQL).

Key Architectural Concepts

1. Double-Hop Access: Expose platform-specific generator and inspector helpers under a unified gateway: $this->db()->sql()->generator() and $this->db()->sql()->inspector().
2. Centralized Introspection: Deprecate from_mysql() methods in Column and Index domain objects, moving introspection results parsing to driver-specific Inspector implementations.
3. AST Node Refactoring: Relocate query AST classes (Clauses, Operands, Operators) under Sql\ namespace and delegate DDL/DML rendering to the platform generator.
4. Operations & Fluent Builder: Extract SELECT query logic into Operations\Select, and introduce a fluent query builder under $this->db()->sql()->select().

Ref: #220, #232, #39