feat(rust): port program timing helpers

Move the implementation of programs/timefn.c into rust/src/timefn.rs. The
file provides the monotonic nanosecond clock (UTIL_getTime, span helpers,
UTIL_waitForNextTick, UTIL_support_MT_measurements) used by the CLI and by
several C test tools. timefn.c remains as a declaration-only shim so the
original source lists and header configuration keep working, and it pins the
ABI with static asserts: UTIL_time_t is returned by value and must stay a
plain 64-bit counter, which the Rust #[repr(C)] mirror also asserts.

Platform selection mirrors the C preprocessor structure: Windows uses
QueryPerformanceCounter, Apple targets use mach_absolute_time, and other
POSIX systems use libc clock_gettime(CLOCK_MONOTONIC). Only the unix path is
exercised by this environment; the Windows and Apple paths are written from
the C source and compile-checked logically but are untested here. The C90
clock() fallback is unreachable on Rust-supported targets, so multi-threaded
measurement support is always reported.

The symbols live in the program-only zstd-cli-rs package, keeping them out
of library builds. Linking that archive into C test binaries surfaced a
structural problem: rustc's local ThinLTO promotes internal symbols across
codegen units, so extracting the timefn object could drag in the zstd_cli
parser object, whose FIO_* externs test binaries cannot satisfy. The parser
is therefore gated behind a new additive `cli` cargo feature (default on).
Program archives build with cli,compression,decompression as before, while
tests/Makefile links a helpers-only archive (rust/target/cli-helpers) built
with --no-default-features, which contains no fileio references at all.

tests/Makefile gains build rules for the helpers archive and adds it as a
prerequisite of every binary that compiles the timefn shim: fullbench(32),
fullbench-lib, fullbench-dll, fuzzer(32), zstreamtest(32/asan/tsan/ubsan),
paramgrill, decodecorpus, and poolTests. Prerequisite order places the
archive after all C objects in `$^` link lines; the known-broken -dll
recipes filter to %.c, so they name the archive explicitly. Original C test
sources are untouched; only link inputs changed.

zstd-cli-rs now depends on libc (already used by the core crate) for
clock_gettime and the Mach timebase bindings.

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; roundtrip echo hello | zstd | zstd -d
- make -C tests fullbench fuzzer zstreamtest paramgrill decodecorpus
  poolTests; ./tests/fullbench -i0; ./tests/fuzzer -i1 --no-big-tests;
  ./tests/poolTests; make -C tests test-rust-lib-smoke
- verified with nm that the helpers archive member defining UTIL_getTime has
  no FIO_*/ZSTD_* undefined references

Refs: rust/README.md

rust/README.md

@@ -59,6 +59,11 @@ zstd ABI:
     so library builds do not acquire program-only dependencies.  The C
     `fileio` backend still owns file opening, safe replacement, sparse writes,
     metadata, and streaming I/O.
+  - `timefn` provides the monotonic nanosecond clock behind `UTIL_time_t`.
+    It also lives in the `cli/` package, but C test binaries (fullbench,
+    fuzzer, zstreamtest, paramgrill, ...) link a helpers-only build of that
+    archive, produced without the package's `cli` feature, because the parser
+    layer requires the C `fileio` backend that tests do not compile.
 
 The optimal block matcher, high-level frame compression, dictionary-building,
 legacy decoding callbacks, and the CLI file-I/O backend are still C. They must
@@ -99,8 +104,9 @@ from `rust/cli` as well:
 ```sh
 cargo clippy --all-targets -- -D warnings
 cargo test --all-targets
-cargo test --no-default-features --features compression --all-targets
-cargo test --no-default-features --features decompression --all-targets
+cargo test --no-default-features --features cli,compression --all-targets
+cargo test --no-default-features --features cli,decompression --all-targets
+cargo test --no-default-features --all-targets
 ```
 
 Then run original compatibility tests from the repository root, starting with