(Aura Designer) Fix health bar indicator colour alpha having no effect in replace mode

[Krathe82]

Summary

• The colour picker's alpha slider had no effect on the health bar tint overlay — ApplyHealthBar read r, g, b from config.color but silently dropped color.a, so the overlay was always fully opaque regardless of the alpha setting.
• Additionally, the underlying health bar texture was unconditionally matched to the AD colour (to prevent class-colour OOR bleedthrough). This cancelled the alpha effect: a semi-transparent overlay over an identical-coloured bar appears fully opaque. Fixed by only matching the texture colour when the overlay itself is fully opaque (a >= 1). When a < 1 the user explicitly wants the underlying bar colour to show through.
• This fix also restricts the texture colour match to replace mode only — in tint mode the underlying bar colour is intentionally visible through the semi-transparent overlay (that is how the blend slider works). This supersedes the Indicators.lua portion of PR (Aura Designer) Fix health bar tint indicator not fading correctly when out of range #60.

Changes

AuraDesigner/Indicators.lua — ApplyHealthBar:

• Read a = color[4] or color.a or 1 and use it as the overlay alpha in replace mode
• Restrict hbTex:SetVertexColor to mode == "replace" and a >= 1

Test plan

• Set up a health bar indicator in Replace mode
• Change the colour alpha to 50% — the overlay should become semi-transparent, showing the health bar colour beneath
• Set alpha back to 100% — the overlay should be fully opaque as before
• Switch to Tint mode — the blend slider should mix the AD colour with the underlying bar (unchanged from existing behaviour)
• OOR with replace mode + full alpha: bar should fade without class-colour bleedthrough
• OOR with replace mode + partial alpha: some class colour visible (expected — user opted in by lowering alpha)

Note: This PR supersedes the Indicators.lua changes in PR #60. The ElementAppearance.lua OOR fade changes in PR #60 are independent and still needed.

(PR by Krathe)

[Krathe82]

Dropping this — the colour picker alpha is inconsistent with how other indicator types handle opacity (squares, bars, and icons all use a separate Alpha slider rather than the colour picker alpha channel). Will flag to Danders: colour alpha currently has no effect for any indicator type in replace or tint mode; the question is whether to add a dedicated Alpha slider or remove alpha from the colour picker for these indicators since Blend % covers the use case.