Require two blank lines between SQL statements

[Maxteabag]

Summary

• keep one blank line as ordinary formatting inside a SQL statement
• require two consecutive blank lines before treating whitespace as an implicit statement separator
• apply the same rule to execution normalization and statement-at-cursor ranges
• support both LF and CRLF input

Fixes #281.

Validation

• statement/transaction/alert/process-worker tests: 79 passed
• single-blank-line regression reproduces the reported LIMIT formatting case
• double-blank-line regressions cover LF, CRLF, and whitespace-only lines
• Ruff and structured autoreview: clean