feat(compress): move trace context reset to Rust

Keep the C-owned trace slot behind an explicit pointer projection so the
Rust trace boundary owns the complete enabled-call lifecycle. Rust now reads
the context, publishes the record, and clears the slot after the callback;
C only supplies private CCtx fields and the weak callback adapter.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo check --tests (rust)
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --tests -- -A clippy::manual-bits -D warnings (rust)
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; make -j1 -C tests test
- Focused trace lifecycle test added; standalone cargo test remains link-blocked by the existing C-owned decompression view symbols.

lib/compress/zstd_compress.c

@@ -7362,7 +7362,7 @@ typedef void (*ZSTD_rust_traceEnd_f)(
         void* context, unsigned long long traceCtx, const void* trace);
 typedef struct {
     void* callbackContext;
-    unsigned long long traceCtx;
+    unsigned long long* traceCtx;
     int streaming;
     unsigned dictionaryID;
     size_t dictionarySize;
@@ -7413,7 +7413,7 @@ void ZSTD_CCtx_trace(ZSTD_CCtx* cctx, size_t extraCSize)
 #if ZSTD_TRACE
     ZSTD_rust_traceEndState state;
     state.callbackContext = cctx;
-    state.traceCtx = cctx->traceCtx;
+    state.traceCtx = &cctx->traceCtx;
     state.streaming = cctx->inBuffSize > 0 || cctx->outBuffSize > 0
             || cctx->appliedParams.nbWorkers > 0;
     state.dictionaryID = cctx->dictID;
@@ -7424,7 +7424,6 @@ void ZSTD_CCtx_trace(ZSTD_CCtx* cctx, size_t extraCSize)
     state.cctx = cctx;
     state.traceEnd = ZSTD_rust_traceEnd_callback;
     ZSTD_rust_traceEnd(&state);
-    cctx->traceCtx = 0;
 #else
     (void)cctx;
     (void)extraCSize;