feat(rust): port benchmark loop and CLI bench mode

Move the implementation of programs/benchfn.c into rust/src/benchfn.rs and
wire benchmark mode (-b/-e/-i) into the Rust CLI frontend, which previously
rejected those options as not yet implemented. `zstd -b1 -i0 FILE` and range
runs like `zstd -b5e6 -i0 FILE` work again, including the synthetic-sample
benchmark when no file is given.

benchfn.rs is a faithful port of the run/timing state machine:
BMK_benchFunction keeps the exact loop accounting (first-loop blockResults
and errorFn checks, dstSize summed on the first loop only, 0xE5 warm-up of
result buffers, nbLoops minimum of 1) and BMK_benchTimedFn keeps the same
convergence behavior (x10 workload growth for short runs, budget-based
nbLoops estimation, runs below half the run budget re-tried rather than
reported, best qualifying run returned). Arithmetic that C leaves to
unsigned wrap-around uses wrapping operations so debug builds cannot panic
where release C would wrap.

ABI notes: BMK_runTime_t and BMK_runOutcome_t are returned by value across
the C boundary and BMK_benchParams_t is passed by value, so all three are
repr(C) mirrors of benchfn.h; their field offsets are pinned by const
asserts in Rust and matching C static asserts in the benchfn.c shim, which
is now declaration-only. BMK_timedFnState_t stays opaque, fits the 64-byte
BMK_timedFnState_shell (compile-time checked), and is malloc/free-managed
so creation and destruction remain interchangeable with C callers.

The CLI parses -b (bench mode), -e (range end, digits attach directly,
defaulting to 0 like readU32FromChar) and -i (duration in seconds), then
dispatches through a new ZSTD_rust_cli_bench bridge in the zstdcli.c shim.
The bridge exists because benchmark availability is a C preprocessor
property (ZSTD_NOBENCH): orchestration and reporting stay in C benchzstd.c,
stripped variants (zstd-small, zstd-compress, zstd-decompress) compile the
stub branch and report "benchmark mode is not available in this build", and
the Rust side never references benchmark symbols directly. Level clamping
against ZSTD_maxCLevel() happens in the bridge, where the symbol is
guaranteed to exist whenever benchmarking is compiled in. -T selects the
worker count, defaulting to single-threaded like the C bench path; -S
(separate files) and --priority=rt remain unimplemented.

Makefile updates only extend the Rust source prerequisite lists with
benchfn.rs; the helpers-archive plumbing from the timefn commit already
links fullbench(-lib/-dll/32) and paramgrill, the benchfn consumers among
the C tests. Original C test sources are untouched.

Known pre-existing issues, unchanged by this commit: tests/fullbench-lib
fails to link at the base commit too (libzstd.a precedes fullbench.c in its
link line), and the cli-tests basic/help.sh, compression/levels.sh,
compression/golden.sh, and decompression/pass-through.sh scripts fail
identically with a base-commit binary because the Rust CLI frontend is
still a partial reimplementation.

Test Plan:
- cd rust && cargo fmt --check && cargo clippy --all-targets -- -D warnings
  && cargo test --all-targets && cargo build --release
- cd rust/cli && cargo fmt --check && cargo clippy --all-targets -- -D
  warnings && cargo test --all-targets; repeat tests with
  --no-default-features plus features compression / decompression / (none)
- make -C programs zstd; ./programs/zstd -b1 -i0 lib/common/xxhash.c;
  ./programs/zstd -b5e6 -i0 programs/fileio.c; ./programs/zstd -b1 -i0
  (synthetic); echo roundtrip via zstd | zstd -d
- make -C programs zstd-small zstd-compress zstd-decompress zstd-nolegacy
  zstd-dictBuilder; zstd-small -b reports benchmark unavailable; compress/
  decompress roundtrip across the split binaries
- make -C tests fullbench fuzzer zstreamtest paramgrill decodecorpus
  poolTests fullbench32 fuzzer32; ./tests/fullbench -i0 (exercises Rust
  BMK_benchTimedFn from C); ./tests/fullbench32 -i0; ./tests/fuzzer -i1
  --no-big-tests; ./tests/poolTests; make -C tests test-rust-lib-smoke
- cli-tests subset: basic/version.sh, compression/basic.sh,
  compression/multiple-files.sh pass; failing scripts match the base commit

Refs: rust/README.md

programs/Makefile

@@ -33,7 +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/timefn.rs
+                    $(RUST_DIR)/src/timefn.rs $(RUST_DIR)/src/benchfn.rs
 
 # Keep Rust's HUF implementation in lockstep with libzstd.mk's C selection.
 # Forced modes may arrive as libzstd.mk variables or as direct -D flags in