[client] clipboard: reduce remote file transfer latency

Mark the last non-empty FILE_DATA payload with BEGIN and END when the
producer knows it is terminal. Complete producer bookkeeping in the
following fileDataEnd call without consuming another LGMP grant. Keep
the standalone terminal record for empty and legacy unknown streams.

Drain up to 64 queued clipboard records per worker pass instead of
sleeping after every message. Poll at 1 ms while transfers, blocked
writes, held input, or pending output are active, and retain the 10 ms
interval while idle.

This removes a serialized grant round trip from every non-empty file
response and reduces scheduling delay while a large transfer is active.
Keep the 1 MiB payload size because the bottleneck was stop-and-wait
latency, not the chunk capacity.

Exercise a full 1 MiB response in a single BEGIN|END grant and retain
coverage for empty unknown-size responses.

client/include/interface/clipboard.h

@@ -247,9 +247,12 @@ typedef struct LG_ClipboardOps
       const LG_ClipboardFileRequest * request);
   LG_ClipboardResult (*fileDataBegin)(void * opaque,
       const LG_ClipboardFileRequest * request, uint64_t sizeHint);
+  /* end marks a final non-empty chunk. fileDataEnd must still be called to
+   * complete the producer-side stream; it emits a terminal record only when
+   * the final chunk was not marked, including for an empty response. */
   LG_ClipboardResult (*fileDataChunk)(void * opaque,
       const LG_ClipboardFileRequest * request, uint64_t responseOffset,
-      const void * data, size_t size);
+      const void * data, size_t size, bool end);
   LG_ClipboardResult (*fileDataEnd)(void * opaque,
       const LG_ClipboardFileRequest * request, uint64_t finalSize);
   bool (*fileCancel)(void * opaque, uint64_t dataset,