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.

src/client/client_cli.c

@@ -476,6 +476,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--copy-unsafe-links", NULL, OPT_FLAG, offsetof(Config, copy_unsafe_links)},
     {"--sparse", "-S", OPT_FLAG, offsetof(Config, preserve_sparse)},
     {"--inplace", NULL, OPT_FLAG, offsetof(Config, inplace)},
+    {"--preallocate", NULL, OPT_FLAG, offsetof(Config, preallocate)},
     {"--append", NULL, OPT_FLAG, offsetof(Config, append)},
     {"--append-verify", NULL, OPT_FLAG, offsetof(Config, append_verify)},
     {"--fsync", NULL, OPT_FLAG, offsetof(Config, use_fsync)},
@@ -551,6 +552,7 @@ static const NegatableOption NEGATABLE_OPTIONS[] = {
     {"copy-unsafe-links", NULL, offsetof(Config, copy_unsafe_links)},
     {"sparse", "S", offsetof(Config, preserve_sparse)},
     {"inplace", NULL, offsetof(Config, inplace)},
+    {"preallocate", NULL, offsetof(Config, preallocate)},
     {"checksum", NULL, offsetof(Config, checksum)},
     {"from0", NULL, offsetof(Config, from0)},
     {"cvs-exclude", NULL, offsetof(Config, cvs_exclude)},