Keep corrupted gem requirements at base level

[unrealdreamz]

Summary

Fixes #1738.

Keeps imported level-corrupted gems using their effective corrupted level for skill calculations, while using the uncorrupted gem level for level/attribute requirements.

Root Cause

Character import folded Levels from Corruption directly into gemInstance.level. SkillsTabClass:ProcessSocketGroup() then used that same effective level to compute reqLevel, reqStr, reqDex, and reqInt, so a +1/-1 corrupted gem incorrectly changed its requirements.

Fix

• Import now preserves the gem's natural requirement level separately as requirementGemLevel when a level corruption is present.
• Requirement calculation uses requirementGemLevel when present, falling back to the effective gem level for normal gems.
• The separate requirement level is saved/loaded with the build so imported characters stay correct after reopening.
• Manual gem level/gem selection/default-level changes clear the import-only override to avoid stale requirement data.
• Gem tooltips use the same requirement-level helper as the skills tab.
• Added a regression covering both +level and -level corruption semantics.

Validation

Local, non-GUI validation only:

gh pr list --repo PathOfBuildingCommunity/PathOfBuilding-PoE2 --state open -S "1738 corrupted skill gem requirements import" -> []
git diff --check -> passed
git diff --cached --check -> passed
git show --check --stat --oneline HEAD -> passed
where.exe lua -> not found
where.exe luajit -> not found
where.exe busted -> not found

I did not run Busted locally because this Windows checkout does not have Lua/LuaJIT/Busted on PATH. I also avoided Docker/GUI validation so no visible command windows or heavy processes were launched.

Risk / Rollback

Moderate-low risk. Normal gems continue to use their effective gem level for requirements. Only imported/saved gems with requirementGemLevel take the alternate path, and manual edits clear it. Rollback is this commit.