feat(cli): port file I/O preferences to Rust

Move fileio preference and context allocation, defaults, setters, and small
query helpers into Rust while keeping the actual file operations in C. Keep
the C structs and ABI declarations as anchors, and make the CLI build track
the new Rust source.

Test Plan:
- rustfmt +nightly --check --edition 2021 rust/src/fileio_prefs.rs rust/cli/src/lib.rs
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo test --manifest-path rust/cli/Cargo.toml
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings
- git diff --cached --check

programs/Makefile

@@ -33,6 +33,7 @@ RUST_SOURCES := $(RUST_MANIFEST) $(RUST_DIR)/Cargo.lock \
                 $(shell find $(RUST_DIR)/src -type f -name '*.rs' -print)
 RUST_CLI_SOURCES := $(RUST_CLI_MANIFEST) $(RUST_CLI_DIR)/Cargo.lock \
                     $(RUST_CLI_DIR)/src/lib.rs $(RUST_DIR)/src/zstd_cli.rs \
+                    $(RUST_DIR)/src/fileio_prefs.rs \
                     $(RUST_DIR)/src/timefn.rs $(RUST_DIR)/src/benchfn.rs \
                     $(RUST_DIR)/src/datagen.rs $(RUST_DIR)/src/lorem.rs