Character-only realtime shadows via a DirectionalLight + a private-layer
CPU-skinned caster proxy; terrain/models/static_gx receive through Bevy's
shadows::fetch_directional_shadow. Opt-in cvar `worldShadows` (default off);
hides the blob oval when on. Branched from 7f4db0e (clean pre-shadow base).
Conflicts resolved: cvars.rs (our `same()` style + 45-entry session array),
options_tests.rs (our row counts +1 for the World Shadows toggle).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
