cuda: add cached_trim() to release pooled GPU buffers

Exposes the caching allocator's trim() through a public free function.
Called after weight fusion during model loading to free temporary buffers
that would otherwise sit in the pool and cause OOM.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-cuda/src/allocator.rs

@@ -100,6 +100,13 @@ pub fn cached_alloc(size: usize) -> Result<GpuBuffer> {
     })
 }
 
+/// Free all cached (unused) GPU buffers back to the driver.
+pub fn cached_trim() {
+    ALLOCATOR.with(|cell| {
+        cell.borrow_mut().trim();
+    });
+}
+
 /// Return a raw GPU pointer to the caching allocator's free list.
 /// Called from `GpuBuffer::Drop` for pooled buffers. Takes raw pointer
 /// and size to avoid re-triggering Drop.