Route the CLI backend's safe file-removal leaf through a narrow Rust status ABI while preserving C diagnostics, Windows read-only handling, and the existing return convention.
Test Plan: cargo test --manifest-path rust/cli/Cargo.toml; cargo clippy --manifest-path rust/cli/Cargo.toml; cargo clippy --manifest-path rust/cli/Cargo.toml --benches; cargo clippy --manifest-path rust/cli/Cargo.toml --tests; make -B -C programs -j2 zstd; make -C tests -j2 test-cli-tests (41 tests); make -B -C programs -j2 zstd-small zstd-frugal.
ddidderr
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