Improve handling of negative offsets in block masks

[aksiome]

🗂️ Request Type

Feature

🧩 Feature or Module Name

#bs.block:fill_*

📝 Description

When using negative offsets in block masks within the block module, the block being tested may have already been modified earlier in the process. This can lead to incorrect results or unintended side effects. To mitigate this, we could, for example, automatically detect negative offsets and adjust behavior accordingly. We could for example pre-test blocks before they are changed. However, any solution must preserve the current single-pass processing constraint to maintain performance and simplicity.