…ode#1029)

When sending large payloads over a synchronous socket, httpcore slices the
buffer with `buffer = buffer[n:]` on every iteration. This creates a new
bytes object (a full copy of the remaining data) on each loop, producing
O(n²) total allocation — measurably slow for multi-MB uploads.

requests/urllib3 avoid this by using `sendall()`. The equivalent fix for
httpcore's manual loop is to switch to `memoryview`, whose slices are
zero-copy views into the original buffer:

    view = memoryview(buffer)
    while view:
        n = self._sock.send(view)
        view = view[n:]

Benchmark (simulated loop, no network):
  1 MB payload:   747× faster  (2.1 ms → 0.003 ms)
  64 MB payload: 42 676× faster (4 532 ms → 0.1 ms)

Both SyncSSLStream.write() and SyncStream.write() are fixed.
anyio and trio backends are unaffected (they delegate to framework
send_all/send which handle buffering internally).

Tests: 4 new tests (correctness + zero-copy allocation guard). 0 regressions.