Detect exposure sources across active skills

[unrealdreamz]

Fixes #996
Refs #561

Summary

• Detect exposure sources from any active player skill, not only the currently selected main skill.
• Keep the existing global exposure-source path intact.
• Add regressions for conditional exposure supports and for a secondary active skill enabling exposure while Spark remains selected.

Root Cause

Exposure configuration eligibility only inspected env.player.mainSkill.skillModList. That made two valid exposure-source cases fail:

1. Conditional support sources such as Lightning Exposure could be hidden because their runtime condition was not already true.
2. Exposure caused by another active skill did not make the exposure configuration valid for the selected damage skill, even though exposure is an enemy state once applied.

Fix

The exposure-source check now scans env.player.activeSkillList for FireExposureChance, ColdExposureChance, or LightningExposureChance mods using HasMod(...), so it detects real support/source mods without requiring their runtime condition to already evaluate true. Global/non-skill exposure chance still uses the existing evaluated modDB:Sum(...) > 0 path.

Validation

• git diff --check - pass.
• git diff --cached --check - pass before amended commit.
• git show --check --stat --oneline --no-renames HEAD - pass.
• python/lupa syntax smoke for spec/System/TestSkills_spec.lua - pass.
• Added targeted regressions in spec/System/TestSkills_spec.lua.
• Full Busted/Docker suite not run locally: docker, docker-compose, lua, luajit, and busted are not installed on PATH on this machine.

Risk / Rollback

Risk is low and contained to exposure config/source eligibility. The change does not force exposure to apply; users still need to enable the exposure configuration, and exposure magnitude still flows through the existing enemy modifier path. Rollback is the single commit if maintainers prefer exposure eligibility to remain main-skill-only.