Emulate UNSIGNED integer semantics for CAST and CONVERT #359

Problem

MySQL's CAST(expr AS UNSIGNED) and CONVERT(expr, UNSIGNED) wrap negative values using modular arithmetic (2^64). For example, CAST('-10' AS UNSIGNED) returns 18446744073709551606 in MySQL.

SQLite has no unsigned integer type — its integers are always 64-bit signed. Currently, CAST('-10' AS UNSIGNED) returns -10 in the SQLite driver, which is incorrect.

Scope

The UNSIGNED semantics may need to be emulated in multiple places:

• CAST(expr AS UNSIGNED) expressions.
• CONVERT(expr, UNSIGNED) expressions.
• Potentially other contexts where UNSIGNED integer behavior is expected (e.g., column types, arithmetic).

References

• TODOs added in Translate MySQL CONVERT() expressions to SQLite #356.