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/usage.c

@@ -169,6 +169,7 @@ void print_usage(void) {
   printf("  --copy-unsafe-links  Only transform unsafe symlinks into referent files\n");
   printf("  -S, --sparse        Handle sparse files efficiently\n");
   printf("  --inplace           Update files in-place (no temp+rename)\n");
+  printf("  --preallocate       Allocate destination file space up front (fail-fast on full disk)\n");
   printf("  --append            Resume a shorter destination by appending only its tail\n");
   printf("                    (prefix is not verified; requires --incremental)\n");
   printf("  --append-verify     Like --append, but verifies the retained prefix checksum\n");