refactor(decompress): move stack heap-mode policy to Rust

Move only the heap-mode branch decision for stack decompression into Rust
while retaining the C-owned stack context, static initialization, error codes,
and decoder projection. The Rust scalar policy preserves the historical
heapmode >= 1 rejection boundary.

Test Plan:
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings` -- passed
- `ulimit -v 41943040; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 make -j1` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 ./tests/rustLibSmoke` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 make -j1 -C tests test` -- passed, including large streaming, native, fuzzer, and zstream phases

lib/decompress/zstd_decompress.c

@@ -138,6 +138,7 @@ void ZSTD_rust_dctx_init_platform(void* bmi2, int dynamic_bmi2);
 size_t ZSTD_rust_dctx_default_max_window_size(void);
 int ZSTD_rust_no_forward_progress_max(void);
 int ZSTD_rust_heapmode(void);
+int ZSTD_rust_decompress_stack_uses_heap(int heapmode);
 size_t ZSTD_rust_decompress_stack(void* dst, size_t dstCapacity,
                                   const void* src, size_t srcSize);
 size_t ZSTD_rust_decompress_stack_context(ZSTD_DCtx* dctx,
@@ -274,19 +275,14 @@ int ZSTD_rust_heapmode(void)
 size_t ZSTD_rust_decompress_stack(void* dst, size_t dstCapacity,
                                   const void* src, size_t srcSize)
 {
-#if ZSTD_HEAPMODE >= 1
-    (void)dst;
-    (void)dstCapacity;
-    (void)src;
-    (void)srcSize;
-    return ERROR(GENERIC);
-#else
+    if (ZSTD_rust_decompress_stack_uses_heap(ZSTD_HEAPMODE)) {
+        return ERROR(GENERIC);
+    }
     ZSTD_DCtx dctx;
     ZSTD_DCtx* const initialized = ZSTD_initStaticDCtx(&dctx, sizeof(dctx));
     if (initialized == NULL) return ERROR(memory_allocation);
     return ZSTD_rust_decompress_stack_context(initialized, dst, dstCapacity,
                                               src, srcSize);
-#endif
 }
 
 void ZSTD_rust_dctx_trace_view(ZSTD_DCtx* dctx,

rust/src/zstd_decompress.rs

@@ -2913,6 +2913,16 @@ mod dctx_trace_tests {
 mod stack_context_tests {
     use super::*;
 
+    #[test]
+    fn stack_heapmode_policy_rejects_one_and_above_but_keeps_zero_and_negative() {
+        for heapmode in [c_int::MIN, -1, 0] {
+            assert_eq!(ZSTD_rust_decompress_stack_uses_heap(heapmode), 0);
+        }
+        for heapmode in [1, c_int::MAX] {
+            assert_eq!(ZSTD_rust_decompress_stack_uses_heap(heapmode), 1);
+        }
+    }
+
     #[repr(C)]
     struct StackDispatchProbe {
         static_size: usize,
@@ -3625,6 +3635,20 @@ unsafe fn decompress_stack_context(
     unsafe { dispatch(view.dctx.cast(), dst, dst_capacity, src, src_size) }
 }
 
+#[inline]
+fn decompress_stack_uses_heap(heapmode: c_int) -> bool {
+    heapmode >= 1
+}
+
+/// Select whether the C stack-decompression path must reject heap mode.
+///
+/// The C caller retains the stack context and static initialization; Rust
+/// owns only the configuration scalar's branch policy.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_decompress_stack_uses_heap(heapmode: c_int) -> c_int {
+    c_int::from(decompress_stack_uses_heap(heapmode))
+}
+
 /// Rust policy/dispatch for the C-owned stack context used by heapmode=0.
 ///
 /// The context remains opaque here; the C projection supplies only the field