Fix MariaDB compatibility issue with multiple TIMESTAMP defaults

[Copilot]

Resolves the MariaDB compatibility error when creating the versionx_delta table during installation.

Problem

MariaDB enforces a restriction where only one TIMESTAMP column per table can have CURRENT_TIMESTAMP in the DEFAULT or ON UPDATE clause. The vxDelta table definition had both time_start and time_end fields configured with default="CURRENT_TIMESTAMP", causing this error:

Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause

Solution

• Removed the CURRENT_TIMESTAMP default from the time_end field in the XML schema
• Updated the corresponding map file to reflect this change
• Kept time_start with the CURRENT_TIMESTAMP default as the primary timestamp field

Impact

• ✅ Maintains full functionality - the application code explicitly sets both time_start and time_end when creating deltas
• ✅ Resolves MariaDB installation errors
• ✅ Preserves existing behavior for MySQL users
• ✅ Minimal change with no breaking changes

The fix ensures compatibility with MariaDB while maintaining the existing delta creation logic where both timestamps are set to the same value ($now) by the application.

Fixes #148.

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