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.

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)]