fix: sponge v3 palette writer with __reserved__

[PierreSchwang]

Overview

Fixes an issue where reserved was written into the block palette for sponge v3 schematics (example would be masked copies).

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.