feat(mt): move compression-job begin policy to Rust

The worker context previously selected the cdict or raw-prefix initialization
path, applied non-first-job parameter updates, and published the frame-header
projection directly in C. That left the branch and failure order intertwined
with private CCtx and parameter layouts.

Project only the first-job and cdict flags into Rust. Rust now validates cdict
placement, selects the initialization path, stops on force-window or
prefix-policy errors, and publishes the header projection only after successful
initialization. C callbacks retain the private cdict, CCtx, parameter mutation,
and frame-header field operations, including the original pledged-size and
force-window behavior.

Focused tests cover cdict ordering, non-first parameter ordering, parameter and
initialization failures, header-publication suppression, and invalid cdict
placement.

Test Plan:
- `rustfmt +nightly --edition 2021 --check rust/src/zstdmt_compress.rs` -- passed.
- Capped GCC syntax-only check for `zstdmt_compress.c` -- passed.
- Capped Clang syntax-only check for `zstdmt_compress.c` -- passed.
- `git diff --cached --check` -- passed.
- Cargo, native builds, fuzzers, and large tests were not run per assignment.

lib/compress/zstdmt_compress.c

@@ -222,6 +222,18 @@ typedef char ZSTDMT_compression_job_projection_layout[
                 == 2 * sizeof(unsigned) + sizeof(void*))
         ? 1 : -1];
 
+typedef struct {
+    unsigned firstJob;
+    unsigned hasCDict;
+} ZSTDMT_RustCompressionJobBeginProjection;
+typedef char ZSTDMT_compression_job_begin_projection_layout[
+        (offsetof(ZSTDMT_RustCompressionJobBeginProjection, firstJob) == 0
+         && offsetof(ZSTDMT_RustCompressionJobBeginProjection, hasCDict)
+                == sizeof(unsigned)
+         && sizeof(ZSTDMT_RustCompressionJobBeginProjection)
+                == 2 * sizeof(unsigned))
+        ? 1 : -1];
+
 typedef struct {
     int status;
     size_t toFlush;
@@ -235,6 +247,7 @@ typedef void (*ZSTDMT_compressionJobVoidFn)(void* opaque);
 typedef ZSTDMT_chunkProcessResult (*ZSTDMT_compressionJobCompressFn)(
         void* opaque, unsigned lastJob);
 typedef void (*ZSTDMT_compressionJobSizeFn)(void* opaque, size_t size);
+typedef size_t (*ZSTDMT_compressionJobSetParameterFn)(void* opaque, int value);
 
 typedef struct {
     void* callbackContext;
@@ -284,6 +297,15 @@ typedef char ZSTDMT_compression_job_parameters_layout[
 ZSTDMT_RustCompressionJobParameters ZSTDMT_rust_prepareCompressionJobParameters(
         unsigned jobID, int checksumFlag, int ldmEnable, unsigned nbWorkers);
 
+size_t ZSTDMT_rust_compressionJobBegin(
+        const ZSTDMT_RustCompressionJobBeginProjection* projection,
+        void* opaque,
+        ZSTDMT_compressionJobStepFn beginWithCDict,
+        ZSTDMT_compressionJobSetParameterFn setForceMaxWindow,
+        ZSTDMT_compressionJobStepFn setDeterministicRefPrefix,
+        ZSTDMT_compressionJobStepFn beginWithPrefix,
+        ZSTDMT_compressionJobVoidFn publishFrameHeader);
+
 void ZSTDMT_rust_compressionJob(
         const ZSTDMT_RustCompressionJobProjection* projection,
         const ZSTDMT_RustCompressionJobFinishProjection* finishProjection,
@@ -1702,36 +1724,79 @@ static void ZSTDMT_compressionJobGenerateSequences(void* opaque)
             ZSTDMT_serialState_advance);
 }
 
+static size_t ZSTDMT_compressionJobBeginWithCDict(void* opaque);
+static size_t ZSTDMT_compressionJobSetForceMaxWindow(void* opaque, int value);
+static size_t ZSTDMT_compressionJobSetDeterministicRefPrefix(void* opaque);
+static size_t ZSTDMT_compressionJobBeginWithPrefix(void* opaque);
+static void ZSTDMT_compressionJobPublishFrameHeader(void* opaque);
+
 static size_t ZSTDMT_compressionJobBegin(void* opaque)
 {
     ZSTDMT_compressionJobState* const state =
         (ZSTDMT_compressionJobState*)opaque;
     ZSTDMT_jobDescription* const job = state->job;
-    size_t initError;
 
-    if (job->cdict) {
-        initError = ZSTD_compressBegin_advanced_internal(
-            state->cctx, NULL, 0, ZSTD_dct_auto, ZSTD_dtlm_fast, job->cdict,
-            &state->jobParams, job->fullFrameSize);
-        assert(job->firstJob);  /* only allowed for first job */
-    } else {
-        U64 const pledgedSrcSize = job->firstJob ? job->fullFrameSize : job->src.size;
-        size_t const forceWindowError = ZSTD_CCtxParams_setParameter(
-                &state->jobParams, ZSTD_c_forceMaxWindow, !job->firstJob);
-        if (ZSTD_isError(forceWindowError)) return forceWindowError;
-        if (!job->firstJob) {
-            size_t const err = ZSTD_CCtxParams_setParameter(
-                &state->jobParams, ZSTD_c_deterministicRefPrefix, 0);
-            if (ZSTD_isError(err)) return err;
-        }
-        DEBUGLOG(6, "ZSTDMT_compressionJob: job %u: loading prefix of size %zu", job->jobID, job->prefix.size);
-        initError = ZSTD_compressBegin_advanced_internal(
-            state->cctx, job->prefix.start, job->prefix.size,
-            ZSTD_dct_rawContent, ZSTD_dtlm_fast, NULL, /*cdict*/
-            &state->jobParams, pledgedSrcSize);
-    }
+    ZSTDMT_RustCompressionJobBeginProjection const projection = {
+        job->firstJob,
+        (unsigned)(job->cdict != NULL)
+    };
+    return ZSTDMT_rust_compressionJobBegin(
+            &projection, state,
+            ZSTDMT_compressionJobBeginWithCDict,
+            ZSTDMT_compressionJobSetForceMaxWindow,
+            ZSTDMT_compressionJobSetDeterministicRefPrefix,
+            ZSTDMT_compressionJobBeginWithPrefix,
+            ZSTDMT_compressionJobPublishFrameHeader);
+}
+
+static size_t ZSTDMT_compressionJobBeginWithCDict(void* opaque)
+{
+    ZSTDMT_compressionJobState* const state =
+        (ZSTDMT_compressionJobState*)opaque;
+    ZSTDMT_jobDescription* const job = state->job;
+
+    return ZSTD_compressBegin_advanced_internal(
+        state->cctx, NULL, 0, ZSTD_dct_auto, ZSTD_dtlm_fast, job->cdict,
+        &state->jobParams, job->fullFrameSize);
+}
+
+static size_t ZSTDMT_compressionJobSetForceMaxWindow(void* opaque, int value)
+{
+    ZSTDMT_compressionJobState* const state =
+        (ZSTDMT_compressionJobState*)opaque;
+
+    return ZSTD_CCtxParams_setParameter(
+            &state->jobParams, ZSTD_c_forceMaxWindow, value);
+}
+
+static size_t ZSTDMT_compressionJobSetDeterministicRefPrefix(void* opaque)
+{
+    ZSTDMT_compressionJobState* const state =
+        (ZSTDMT_compressionJobState*)opaque;
+
+    return ZSTD_CCtxParams_setParameter(
+            &state->jobParams, ZSTD_c_deterministicRefPrefix, 0);
+}
+
+static size_t ZSTDMT_compressionJobBeginWithPrefix(void* opaque)
+{
+    ZSTDMT_compressionJobState* const state =
+        (ZSTDMT_compressionJobState*)opaque;
+    ZSTDMT_jobDescription* const job = state->job;
+    U64 const pledgedSrcSize = job->firstJob ? job->fullFrameSize : job->src.size;
+
+    DEBUGLOG(6, "ZSTDMT_compressionJob: job %u: loading prefix of size %zu", job->jobID, job->prefix.size);
+    return ZSTD_compressBegin_advanced_internal(
+        state->cctx, job->prefix.start, job->prefix.size,
+        ZSTD_dct_rawContent, ZSTD_dtlm_fast, NULL, /*cdict*/
+        &state->jobParams, pledgedSrcSize);
+}
+
+static void ZSTDMT_compressionJobPublishFrameHeader(void* opaque)
+{
+    ZSTDMT_compressionJobState* const state =
+        (ZSTDMT_compressionJobState*)opaque;
 
-    if (ZSTD_isError(initError)) return initError;
     state->frameHeaderState.stage = (int*)&state->cctx->stage;
     state->frameHeaderState.noDictIDFlag =
             state->cctx->appliedParams.fParams.noDictIDFlag;
@@ -1747,7 +1812,6 @@ static size_t ZSTDMT_compressionJobBegin(void* opaque)
     state->frameHeaderState.dictID = state->cctx->dictID;
     state->frameHeaderState.repCodes = state->cctx->blockState.prevCBlock == NULL
             ? NULL : state->cctx->blockState.prevCBlock->rep;
-    return initError;
 }
 
 static void ZSTDMT_compressionJobApplySequences(void* opaque)