CRITICAL F003 fixed: netstack/src/scheme/mod.rs had 2 unsafe
File::from_raw_fd sites with redundant 'as RawFd' casts. The 'as
RawFd' cast was a no-op on platforms where RawFd = i32 but was
misleading (suggests a conversion). Removed the cast. Also replaced
generic boilerplate SAFETY comments with specific invariants.
CRITICAL progress: 13 of 13 original findings now addressed
(F001, F1.6, F1.1, F2, F3, DEF-P0-6, DEF-P0-7, F18/F18b, F20,
F21, F3.1, F22, P001).
vasilito
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