Increase file transfer buffer from 512 to 16384 bytes

The 512-byte buffer size results in excessive system calls when serving
files. For a 1MB file, this requires 2048 read/write operations.

Using 16KB reduces system call overhead by 32x and better matches typical
OS page sizes and network buffer defaults. This should improve throughput
for static file serving while maintaining reasonable stack usage.

Author: vlasky

llamafile/server/client.cpp

@@ -784,7 +784,7 @@ Client::dispatcher()
     should_send_error_if_canceled_ = false;
     if (!send(std::string_view(obuf_.p, p - obuf_.p)))
         return false;
-    char buf[512];
+    char buf[16384];
     size_t i, chunk;
     for (i = 0; i < size; i += chunk) {
         chunk = size - i;