List desecrated mods in Add Modifier

[unrealdreamz]

Summary

Fixes #1772.

Adds Desecrated as an Add Modifier source for item bases that can actually spawn those modifiers, so Kurgal/Abyss desecrated mods that already parse from pasted/imported items can also be selected manually from the Craft -> Add Modifier popup.

Root Cause

ItemsTabClass:AddCustomModifierToDisplayItem() only built source lists for normal prefixes/suffixes, essences, and freeform custom text. Desecrated modifiers were loaded in data.itemMods.Desecrated and pasted/imported items already preserved {desecrated} lines, but the Add Modifier popup never exposed that data source.

Fix

• Factored the Add Modifier list construction into ItemsTabClass:BuildAddModifierList(sourceId) so it can be regression-tested without opening UI.
• Added a DESECRATED source that iterates data.itemMods.Desecrated and reuses the existing item spawn-weight filter.
• Marks selected desecrated modifier lines with desecrated = true, preserving the existing raw item round-trip behaviour.
• Added a regression test for ring and body armour Kurgal modifiers.

Validation

Local, non-GUI validation only:

gh pr list --repo PathOfBuildingCommunity/PathOfBuilding-PoE2 --state open -S "1772 desecrated add modifier kurgal" -> []
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

Low risk. This only adds a new Add Modifier source using already-loaded desecrated mod data and existing spawn-weight filtering. If needed, rollback is just this commit; pasted/imported desecrated item parsing remains unchanged.

[LocalIdentity]

Already handled by this PR #1773