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outcomes are only sent after publication so skipped/unpublished sources are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/client/usage.c

@@ -25,6 +25,7 @@ void print_usage(void) {
   printf("  -8, --8-bit-output  Leave high-bit characters unescaped in output\n");
   printf("  --delete            Delete files on receiver not in source\n");
   printf("  --ignore-existing  Skip files that already exist on receiver\n");
+  printf("  --delay-updates    Put updated files into place only at the end of transfer\n");
   printf(
       "  --dirs, --old-dirs, --old-d  Transfer directories without recursing (not implemented)\n");
   printf("  --del               Alias for --delete-during (not implemented)\n");