feat: implement --delay-updates receiver staging and publication

Stage every successfully written file under a private 0700 .fastsync-stage
directory inside the receive root and atomically publish all staged files
only after the whole protocol stream (manifest/delete handling included)
has completed, immediately before the success/outcome frame.  On any
abort/error before publication nothing is installed and staging is removed;
a publish failure aborts the transfer with best-effort cleanup of the
remainder (already-published files are not rolled back).  Crash leftovers
are wiped when the next delayed transfer starts.

Wire: new delay_updates config flag (selection-options block), protocol
version bumped to 2.6.0, client/server validation rejects --inplace.
CLI/usage/validation updated.  Works in single-threaded and -m modes
(exactly one write_thread stages files; the staged-file registry is
mutex-protected; publication runs once after both threads join).
--existing/--ignore-existing/--update decide against the final destination
at stage time; --backup is deferred to publication.  remove_source_files
outcomes are only sent after publication so skipped/unpublished sources are
never deleted.  Default (no flag) behavior is unchanged.

Tests: config wire round-trip, CLI parse, --inplace rejection, new
test_delay_updates unit suite (27 suites total), and integration
TestDelayUpdates covering single/-m parity, incremental reruns, remove
source files, receiver-skip ordering, and a deterministic publish-failure
abort path.

src/shared/config.h

@@ -8,6 +8,10 @@
 
 typedef enum { TRANSPORT_TCP, TRANSPORT_SSH } TransportType;
 
+/* Receiver-side staging state for --delay-updates.  Forward-declared here so
+   Config can carry it; the concrete type lives in delay_updates.h. */
+typedef struct DelayUpdatesContext DelayUpdatesContext;
+
 typedef struct Config {
   char* version;
   char* send_directory;
@@ -91,6 +95,7 @@ typedef struct Config {
   bool ignore_existing;
   bool update;
   bool inplace;
+  bool delay_updates;
   bool use_fsync;
   bool append;
   bool append_verify;
@@ -147,9 +152,13 @@ typedef struct Config {
   char** skip_compress_suffixes;
   int skip_compress_count;
   bool skip_compress_set;
+
+  // Receiver-side runtime staging registry for --delay-updates.  Never sent
+  // over the wire and never set on the sender side.
+  DelayUpdatesContext* delay_context;
 } Config;
 
-#define PROTOCOL_VERSION "2.5.0"
+#define PROTOCOL_VERSION "2.6.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(void);