Optimize SQL script splitting hot paths

[cofin]

Summary

SQL script splitting should avoid unnecessary repeated setup and quadratic string/token accumulation costs on large scripts.

Scope

• Reuse splitter instances by dialect and trailing-terminator behavior.
• Materialize token lists for predictable compiled hot-path behavior.
• Keep statement splitting semantics unchanged across supported dialects.
• Preserve StatementSplitter._tokenize as an internal compatibility surface covered by tests.

Validation

Covered by core splitter unit tests and the core unit suite in PR #545.