[Brollyy]

Summary

• Add backward-compatible ease_in and ease_out properties to fade-based overlay, filter, pixelation, and zoom effects.
• Centralize boundary envelope evaluation in EffectEnvelope.
• Preserve the existing output when both properties are omitted or left enabled.
• Document the new properties for each supported effect.

Behavior

• ease_in = false starts the effect at full strength.
• ease_out = false keeps the effect at full strength until the event ends.
• Missing properties default to true, so existing events are unchanged.

Validation

• dotnet build BopVisualEffects.sln -p:GameRoot=... -p:BepInExPluginsDir= --no-restore
• Build result: 0 warnings, 0 errors.
• git diff --check

Closes #33.