Scale Force of Will by missing mana

[unrealdreamz]

Refs #258

Summary

• Scale Force of Will's Arcane Surge effect by missing mana instead of treating the full line as a flat 20% increased effect.
• Add a missing-mana multiplier derived from the existing Current Mana % config value.
• Add a regression spec for the exact passive text.

Root Cause

The modifier parser recognized 20% increased Effect of Arcane Surge on you and left per ten percent missing Mana as unparsed trailing text in ModCache.lua. That made Force of Will contribute a flat 20% Arcane Surge effect regardless of current mana.

Fix

• Added a special parser rule for increased effect of arcane surge on you per ten percent missing mana.
• Added Multiplier:MissingManaPercentage beside the existing clamped Multiplier:CurrentManaPercentage config value.
• Tagged the parsed mod with div = 10, so it uses full ten-percent chunks through the existing multiplier semantics.
• Updated the generated cache entry for this exact modifier and added a fixture-style regression in TestItemMods_spec.lua.

Validation

• gh auth status -> authenticated as unrealdreamz.
• Open PR search for this bug family -> no duplicate open PR found.
• git diff --check -> passed; Git only reported existing Windows LF-to-CRLF checkout warnings.
• git diff --cached --check -> passed; Git only reported existing Windows LF-to-CRLF checkout warnings.
• git show --check --stat --oneline --no-renames HEAD -> passed.
• Targeted Busted spec was added but could not be executed locally because this Windows environment has no lua, luajit, busted, docker, or docker-compose on PATH.

Risk / Rollback

Risk is limited to modifiers that explicitly use the new missing-mana multiplier and to the existing Current Mana % config path. Rollback is the single commit if CI finds a parser/cache mismatch.

[LocalIdentity]

Bug with the game, not PoB