v1.2.0

Release v1.2.0: occluded grouped layers, WebP, draft button

Bump 1.1.0 -> 1.2.0 (blender_manifest.toml + bl_info + ADDON_VERSION). Since v1.1.0:
optional layers occluded by the scenery (holdout) and groupable via armature/collection,
lossless WebP for UV and JPEG/WebP for light/shadow, "Render Draft (fast preview)" button,
single "Mark as Optional Layer" button, effective export resolution + stale-output cleanup
in the manifest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>