Fix negative stat item sorting

[unrealdreamz]

Fixes #848

Summary

• Preserve negative measured item scores while sorting unique items by defensive/taken-damage stats.
• Add a regression that verifies an item whose best score is still negative can sort ahead of a weaker baseline item.

Root Cause

ItemDBControl:ListBuilder initialized every stat-sort score to 0 before evaluating replacement slots. For stats such as Taken Fire Damage, the sort transform intentionally negates the output so lower damage is better. When all valid candidates were negative, m_max(0, negativeScore) kept every item at 0, producing a full stat tie and falling back to name sorting.

Fix

Initialize measuredPower as nil for each item and only fall back to 0 after all valid replacement slots have been evaluated. This keeps real negative scores, while still giving unsortable/no-slot items the old neutral fallback.

Validation

• git diff --check - pass, exit code 0.
• Targeted regression added in spec/System/TestItemMods_spec.lua.
• Docker/Busted not run locally in this pass because this machine was explicitly kept free of local test/container windows; GitHub Actions should run the normal suite on the PR.

Risk/Rollback

Low risk: the change is restricted to item database stat sorting. Rollback restores the previous zero-initialization behavior, but would reintroduce name-sorting for all-negative stat scores.