preallocate: --preallocate allocates dest space up front

Receiver allocates the destination file's full size before streaming data
(posix_fallocate preferred, ftruncate fallback on EOPNOTSUPP/ENOSYS) so an
out-of-space transfer fails fast instead of partway. Additive config bool
crossing the wire; PROTOCOL_VERSION 2.10.0 -> 2.11.0. Threaded through all
store paths (atomic, inplace, partial/delay-updates staging, link-dest copy
fallback). Review hardening: explicit lseek(0) before the data write so
correctness does not depend on posix_fallocate leaving the fd offset unchanged.

tests/integration/test_ssh.py

@@ -137,3 +137,7 @@ class TestSSHFeatures:
         r = _run_ssh_test("SSH Exclude (--exclude small.txt)",
                           ["--exclude", "small.txt"], expected_missing=["small.txt"])
         assert r["status"] == "Success", r["error"]
+
+    def test_preallocate(self):
+        r = _run_ssh_test("SSH Preallocate (--preallocate)", ["--preallocate"])
+        assert r["status"] == "Success", r["error"]