feat(mt): move serial sequence turn policy into Rust

Move MT serial turn/skip control and the ordering of LDM generation,
checksum updates, and turn advancement into the Rust rewrite. Keep the C
serial mutexes, LDM window/hash state, checksum state, and codec callbacks
behind a narrow callback bridge so the synchronization and private layouts
remain unchanged. Add focused tests for skipped predecessors, empty LDM
turns, and LDM-before-checksum ordering.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml serial_turn_ -- --nocapture
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040 make -B -C programs -j1 zstd
- ulimit -v 41943040 make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s

rust/README.md

@@ -97,8 +97,10 @@ zstd ABI:
     job-creation decisions, compression-job stage sequencing and error flow,
     pending-output decisions through scalar job projections, and frame
     progression's job-ring scan, error normalization, and active-worker
-    accounting.  Rust also owns frame-block preparation ordering; C callbacks
-    retain the private match-state window and workspace operations.
+    accounting.  Rust also owns frame-block preparation ordering and the MT
+    serial turn/skip policy, including LDM-before-checksum sequencing; C
+    callbacks retain the private match-state window, workspace operations,
+    synchronization, LDM state, and checksum state.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
 - Legacy decoding
@@ -149,7 +151,8 @@ shared- and separate-destination decompression scheduling,
 single-threaded stream initialization and the buffered/stable stream state
 machine, MT stream initialization, MT outer scheduling and flush policy, MT
 compression-job stage sequencing and error flow, MT frame-progression job
-aggregation, frame-block preparation ordering, public sequence-API
+aggregation, frame-block preparation ordering, MT serial turn/skip and
+LDM/checksum sequencing, public sequence-API
 orchestration, sequence-store and block policy, external-producer invocation
 and success-path validation, external-sequence-store reset,
 external-sequence/literals block loop, optional-format decompression loops,