MatterViz WebView preview 16

MatterViz/WebView preview for manual validation of the original Multiwfn GUI parity phase. Includes Linux, macOS and Windows packages with the MatterViz frontend, native Rust WebView/session host and Multiwfn executable. Python runtime artifacts are rejected from every package. CI verifies formal in-memory session bootstrap on all three platforms and real uncached orbital 43 through native major-2 binary transport without runtime session/control/Cube artifacts on extracted Windows and Linux packages. macOS also runs the native control/picker protocol suite and extracted Multiwfn checks; real WKWebView orbital readiness requires an interactive macOS desktop and remains part of manual preview validation. Cube fallback is diagnostic-only and requires MULTIWFN_MATTERVIZ_ALLOW_CUBE_FALLBACK=1. Linux requires glibc 2.35 or newer and a WebKitGTK 4.1 desktop environment.