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/shared/config.h

@@ -124,6 +124,11 @@ typedef struct Config {
   bool use_fsync;
   bool append;
   bool append_verify;
+  /* --preallocate: allocates the destination file's full expected space up
+   * front (before any data is written) so a transfer that would overflow disk
+   * fails fast at allocation time and the file is laid out contiguously,
+   * avoiding fragmentation.  Receiver-side, crosses the wire. */
+  bool preallocate;
 
   // Issue #128: Extended delete options
   /* --delete-excluded: also delete destination entries that were excluded on
@@ -257,7 +262,7 @@ typedef struct Config {
   DelayUpdatesContext* delay_context;
 } Config;
 
-#define PROTOCOL_VERSION "2.10.0"
+#define PROTOCOL_VERSION "2.11.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 /* Upper bound on total basis-dir entries (rsync caps --link-dest at 20). */
 #define MAX_BASIS_DIRS 64