Move Relational API to BindStatement - #697

Core is replacing its C++ Relation classes with one primitive, ClientContext::BindStatement. The Python relational API was built on those classes, bound eagerly through them, and could not express a lazy parameterised relation. This PR replaces that layer. It's a port that also changes behavior.

A relation is now a value in Python: a step record, its inputs, a schema bound once when the step is built, its parameters, and handles on the Python objects it reads. It runs nothing until a terminal. Every step renders into one SELECT while SQL clause order allows, otherwise it closes that SELECT as a common table expression; sql_query() is the text that runs. Names and types are whatever the engine's binder says. User SQL fragments are validated by the engine's parser and spliced as written.

What changes for users:

• A bare string operand of an expression is a value, not a column (col("s") == "x" compares with the text). A warning for one release when it matches a column name.
• A parameterised sql() is lazy and binds its values on every read.
• After a join, column names are the engine's; an input alias stops resolving once a step started a new SELECT.
• distinct().project() does what it says; a filter after a limit applies after the limit.
• rel.query(name, sql) leaves no temporary view behind.
• A SELECT the engine classifies as writing (nextval) runs at the call.
• A persistent view over held rows (materialize(), values()) is refused; use rel.create() or con.register().
• New verbs: with_columns, drop, rename, unnest, unpivot, pivot, sample, head, offset, union_by_name, group_by().agg(), join(suffix=).

Deleted: DuckDBPyRelation and the expression classes in C++, every include of the engine's relation headers.

Notes:

• Construction costs about 0.3 ms per step, mostly the 2.0 parser; data paths are unchanged.
• The behavior contract with every deliberate difference is BEHAVIOR.md on the branch.

Next PRs:

• Refuse a DB-API call that re-enters a fetch on the same connection.
• One value conversion path through the native converter.