Player Outline Through Blocks

Initial 26.3 multi-loader build.

Adds a configurable client-side outline for other players hidden by blocks. Includes separate production builds for Fabric, Forge, and NeoForge. Creates config/player_outline.json on first launch. Shows a bright outline around the player while keeping the skin and body fill hidden behind blocks. Tested with Afterburner 0.3.1 and Complementary Reimagined r5.9.3 on all three loaders. Shader-loader compatibility beyond the tested combination remains under evaluation; see the test report.