[FEATURE] Sky authoring seams: sun direction, radius guard, preset crossfade

[Noisemaker111]

Three sky gaps found during the flagship art pass:

• SkyEnvironmentConfig has no sun azimuth/elevation — the dome's glow comes from the preset (day = bearing 0, 68° up), so a game whose lighting key sits elsewhere (the-robots: 29°/bearing 129°) gets a dome sun that disagrees with its shadows.
• sky.radius past the camera far plane silently depth-clips the dome to black; a dev-mode warning would have saved four capture rounds (wreckway).
• zenithColor/horizonColor under timeOfDay: true only set the noon keyframe and crossfade with engine dawn/dusk presets at other times — authored tropical blue silently rendered pink (vice-isle). Needs a doc line or dev warning.

Acceptance:

• Authorable sun direction consumed by dome glow (and optionally the lighting rig default).
• Dev-mode warnings for the radius and crossfade cases.