Rust userinfo lookup and buffer ownership

RE-895. Reproduce with python3 scripts/analyze_rust_info_lookup.py; focused validation: cargo xtask test-group rust-info-lookup. Machine report.

Rust matches96 original70503 key/value and selector cases, preserving first-match case folding, incomplete-pair rejection, empty values, embedded NUL and null early exits. Oversized logical inputs reject without changing the selector. Native userinfo retrieval, raw pointer aliases and fatal semantics remain open.

Original executable/native runtime and complete Linux/macOS Rust/Bevy/Solari acceptance remain unverified.