fix(dv): reject negative deletion-vector positions in DVWriter.Add

[fallintoplace]

Summary

• Validate DVWriter.Add positions are non-negative.
• Change DVWriter.Add to return an error on negative positions.
• Plumb error handling where DVWriter.Add is called.
• Add regression coverage for negative position rejection in dv_writer tests.

Testing

• go test ./table/dv -run 'TestDVWriter'
• go test ./table -run 'TestScanPruningWithDeletionVector|TestReadTaskDeletionVectorSupersedesPositionalDeletes|TestRewriteDataFilesPreservesSiblingDeletionVector'
• go test ./table ./table/dv

[zeroshade]

LGTM. Rejecting negative positions at the DVWriter.Add entry point before mutating writer state is correct, the production caller now propagates the error, and the negative/zero cases are well covered.