feat(decompress): move trace-record policy into Rust

Keep weak-symbol lookup and the conditional traceCtx/DCtx layout in the C
adapter, but project only the trace slot, callback pointers, context pointer,
and dictionary metadata into Rust. Rust now decides begin/end eligibility and
builds the ABI-compatible trace record, preserving null callbacks, streaming
and size conversion semantics. Existing call sites remain in their
success-only branches, so checksum/error paths still emit no trace end.

Test Plan:
- rustfmt --edition 2021 --check rust/src/zstd_decompress.rs
- git diff --check and git diff --cached --check
- Static rg/diff inspection of weak callbacks, traceCtx lifecycle, dictionary
  metadata, and success-only trace-end call sites
- No cargo, make, native, fuzz, or heavy tests run by request

lib/decompress/zstd_decompress.c

@@ -5,7 +5,8 @@
  * optional members vary with the build configuration and the block decoder
  * shares that object.  This translation unit is therefore a deliberately
  * narrow ABI adapter.  It projects field addresses to Rust while retaining
- * the private layout and configuration-bound legacy and trace leaves.
+ * the private layout and configuration-bound legacy leaves.  Weak trace-symbol
+ * discovery also stays here; Rust receives only a narrow trace projection.
  */
 
 #define ZSTD_STATIC_LINKING_ONLY
@@ -105,7 +106,23 @@ typedef struct {
     size_t dctx_size;
 } ZSTD_rustDctxView;
 
+typedef U64 ZSTD_rustTraceCtx;
+typedef ZSTD_rustTraceCtx (*ZSTD_rustTraceBeginFn)(const void* dctx);
+typedef void (*ZSTD_rustTraceEndFn)(ZSTD_rustTraceCtx traceCtx,
+                                    const void* trace);
+
+typedef struct {
+    ZSTD_rustTraceCtx* trace_ctx;
+    ZSTD_rustTraceBeginFn begin;
+    ZSTD_rustTraceEndFn end;
+    const void* dctx;
+    const void* ddict;
+    int dictionary_is_cold;
+} ZSTD_rustDctxTraceView;
+
 void ZSTD_rust_dctx_view(ZSTD_DCtx* dctx, ZSTD_rustDctxView* out);
+void ZSTD_rust_dctx_trace_view(ZSTD_DCtx* dctx,
+                               ZSTD_rustDctxTraceView* out);
 size_t ZSTD_rust_dctx_sizeof(void);
 void ZSTD_rust_dctx_init_platform(ZSTD_DCtx* dctx);
 size_t ZSTD_rust_dctx_default_max_window_size(void);
@@ -117,11 +134,25 @@ ZSTD_DDict* ZSTD_rust_create_ddict(const void* dict, size_t dictSize,
                                     ZSTD_dictLoadMethod_e dictLoadMethod,
                                     ZSTD_dictContentType_e dictContentType,
                                     ZSTD_customMem customMem);
-void ZSTD_rust_dctx_trace_begin(ZSTD_DCtx* dctx);
-void ZSTD_rust_dctx_trace_end(ZSTD_DCtx* dctx, U64 uncompressedSize,
-                              U64 compressedSize, int streaming);
 unsigned ZSTD_rust_legacy_support(void);
 
+#if ZSTD_TRACE
+static ZSTD_rustTraceCtx ZSTD_rust_trace_begin_callback(const void* dctx)
+{
+    return (ZSTD_trace_decompress_begin != NULL)
+        ? ZSTD_trace_decompress_begin((const ZSTD_DCtx*)dctx) : 0;
+}
+
+static void ZSTD_rust_trace_end_callback(ZSTD_rustTraceCtx traceCtx,
+                                          const void* trace)
+{
+    if (ZSTD_trace_decompress_end != NULL) {
+        ZSTD_trace_decompress_end((ZSTD_TraceCtx)traceCtx,
+                                  (const ZSTD_Trace*)trace);
+    }
+}
+#endif
+
 void ZSTD_rust_dctx_view(ZSTD_DCtx* dctx, ZSTD_rustDctxView* out)
 {
     ZSTD_memset(out, 0, sizeof(*out));
@@ -260,40 +291,19 @@ ZSTD_DDict* ZSTD_rust_create_ddict(const void* dict, size_t dictSize,
                                      dictContentType, customMem);
 }
 
-void ZSTD_rust_dctx_trace_begin(ZSTD_DCtx* dctx)
+void ZSTD_rust_dctx_trace_view(ZSTD_DCtx* dctx,
+                               ZSTD_rustDctxTraceView* out)
 {
+    ZSTD_memset(out, 0, sizeof(*out));
+    out->dctx = dctx;
 #if ZSTD_TRACE
-    dctx->traceCtx = (ZSTD_trace_decompress_begin != NULL)
-                   ? ZSTD_trace_decompress_begin(dctx) : 0;
-#else
-    (void)dctx;
-#endif
-}
-
-void ZSTD_rust_dctx_trace_end(ZSTD_DCtx* dctx, U64 uncompressedSize,
-                              U64 compressedSize, int streaming)
-{
-#if ZSTD_TRACE
-    if (dctx->traceCtx && ZSTD_trace_decompress_end != NULL) {
-        ZSTD_Trace trace;
-        ZSTD_memset(&trace, 0, sizeof(trace));
-        trace.version = ZSTD_VERSION_NUMBER;
-        trace.streaming = streaming;
-        if (dctx->ddict) {
-            trace.dictionaryID = ZSTD_getDictID_fromDDict(dctx->ddict);
-            trace.dictionarySize = ZSTD_DDict_dictSize(dctx->ddict);
-            trace.dictionaryIsCold = dctx->ddictIsCold;
-        }
-        trace.uncompressedSize = (size_t)uncompressedSize;
-        trace.compressedSize = (size_t)compressedSize;
-        trace.dctx = dctx;
-        ZSTD_trace_decompress_end(dctx->traceCtx, &trace);
-    }
-#else
-    (void)dctx;
-    (void)uncompressedSize;
-    (void)compressedSize;
-    (void)streaming;
+    out->trace_ctx = &dctx->traceCtx;
+    out->begin = (ZSTD_trace_decompress_begin != NULL)
+        ? ZSTD_rust_trace_begin_callback : NULL;
+    out->end = (ZSTD_trace_decompress_end != NULL)
+        ? ZSTD_rust_trace_end_callback : NULL;
+    out->ddict = dctx->ddict;
+    out->dictionary_is_cold = dctx->ddictIsCold;
 #endif
 }