docs: document Compiler shared-state semantics (attrs.evolve + _subqueries)

[dtsong]

Problem

base.py:260 uses attrs.evolve(compiler, root=False) which produces a shallow copy. Child compilers share _subqueries, _table_context, and _counter with their parent. This is intentional — CTEs bubble up from nested compiles to the root WITH clause — but completely undocumented.

Any future change assuming evolve gives isolation will introduce bugs.

Scope

Add docstring/comments to:

• Compiler class explaining the shared-state contract
• attrs.evolve call site explaining why shallow copy is intentional
• _subqueries dict explaining the CTE bubbling mechanism

Key Files

• data_diff/databases/base.py:127 (_subqueries)
• data_diff/databases/base.py:260 (evolve call)

Acceptance Criteria

• Compiler shared-state semantics documented in code
• Future contributors understand the isolation boundary