CCL_Clay3DP v1.2.0-alpha

Settings UI redesign (3-col landscape); Material section expansion (Water %, Shrinkage); build-volume safety; auto-rebuild on settings + geometry change; Skirt + Base in Preview; elliptical-pipe preview; ARCHITECTURE.md added. NOTE: No dist bundle was published for this release — superseded same-day by v1.2.1-alpha (same content + panel-leak fix). See CHANGELOG.md for details.