Residual pixels remain when overwriting half-block characters via partial WriteConsoleOutputW in conhost.exe

### Windows Terminal version

_No response_

### Windows build number

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### Other Software

_No response_

### Steps to reproduce

See attachment

[conhost_halfblock_artifact_report.txt](https://github.com/user-attachments/files/20259878/conhost_halfblock_artifact_report.txt)

### Expected Behavior

See attachment

### Actual Behavior

See attachment

Start with conhost.exe and the attached conhost_halfblock_artifact_report.txt, then trace the partial WriteConsoleOutputW path described in the reproduction. Confirm the half-block overwrite case and identify why residual pixels remain. Done means the reported artifact no longer appears when half-block characters are overwritten.

Written by the indexing model from the issue text.