fix(mt): keep projected job snapshots under mutex

The Rust-owned frame-progression ring scan consumes ZSTDMT_jobProjection
through ZSTDMT_projectJob. That callback previously copied counters under
job_mutex but read source and prefix metadata after unlocking. Keep the
complete projection in one mutex-protected snapshot, matching the original
ZSTDMT_getFrameProgression access pattern without moving descriptor or
synchronization ownership out of C.

Test Plan:
- `rustfmt --check --edition 2021 rust/src/zstdmt_compress.rs` -- passed.
- `git diff --check` and `git diff --cached --check` -- passed.
- Static inspection only; no cargo, make, native binaries, tests, fuzzers,
  or heavyweight commands were run.

lib/compress/zstdmt_compress.c

@@ -1703,12 +1703,12 @@ static void ZSTDMT_projectJob(void* opaque, unsigned jobID,
     ZSTD_pthread_mutex_lock(&job->job_mutex);
     projection->consumed = job->consumed;
     projection->cSize = job->cSize;
-    projection->dstFlushed = job->dstFlushed;
-    ZSTD_pthread_mutex_unlock(&job->job_mutex);
     projection->srcStart = job->src.start;
     projection->srcSize = job->src.size;
     projection->prefixStart = job->prefix.start;
     projection->prefixSize = job->prefix.size;
+    projection->dstFlushed = job->dstFlushed;
+    ZSTD_pthread_mutex_unlock(&job->job_mutex);
 }
 
 static void ZSTDMT_freeJobsTable(ZSTDMT_jobDescription* jobTable, U32 nbJobs, ZSTD_customMem cMem)

rust/src/zstdmt_compress.rs

@@ -664,8 +664,8 @@ pub type ZSTDMT_bufferGetFn = unsafe extern "C" fn(*mut c_void) -> ZstdMtBuffer;
 
 /// Scalar snapshot of one C-owned job descriptor.
 ///
-/// The C adapter fills this projection while holding the descriptor mutex.
-/// Rust owns only the ring-scan decisions below; the descriptor layout,
+/// The C adapter fills the complete projection while holding the descriptor
+/// mutex. Rust owns only the ring-scan decisions below; the descriptor layout,
 /// synchronization objects, and all pointer ownership stay in C.
 #[repr(C)]
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]