refactor: decompose base.py (1,315 lines) into compiler/dialect/database modules

[dtsong]

Problem

data_diff/databases/base.py (1,315 lines) bundles four distinct responsibilities:

1. Compiler — SQL AST compilation
2. BaseDialect — SQL dialect abstraction
3. Database — Connection management (cloud/async)
4. ThreadedDatabase — Connection pooling (cursor-based)

Every driver author must understand the entire file. This is a barrier to contribution.

Proposed Decomposition

data_diff/databases/
  compiler.py       # Compiler class
  dialect.py        # BaseDialect ABC
  database.py       # Database + ThreadedDatabase ABCs
  base.py           # Re-exports for backward compatibility (temporary)

Constraints

• All existing imports must continue working
• No behavioral changes
• Driver files should not need modification

Acceptance Criteria

• base.py split into 3-4 focused modules
• All existing tests pass without modification
• Backward-compatible re-exports in base.py