feat(delete): default --delete to rsync delete-during; add --delete-commit

- plain --delete with no timing flag now selects delete-during (progressive
  deletion, matching rsync and avoiding the full old+new tree peak)
- new long-only FastSync --delete-commit restores the old atomic behavior
  (delete only after the whole transfer succeeds); timing-identical to
  --delete-after, implemented via the same wire bool
- timing flags are mutually exclusive; --delete-commit conflicts with other
  timings; --delete-before/--delete-during rows reworded per Phase-0 probes
- CHANGELOG migration note; tally unchanged 116/14/27

tests/test_delete_plan.c

@@ -17,6 +17,7 @@
  * caller/receiver entry point) describing `dir` with no kept children. */
 static void send_plan_frame(int fd, const char* dir) {
   EXPECT_TRUE(send_int(fd, 0)); /* has_config */
+  EXPECT_TRUE(send_int(fd, 1)); /* apply: a real plan */
   EXPECT_TRUE(send_wire_str(fd, dir));
   EXPECT_TRUE(send_int(fd, 0)); /* kept child dirs */
   EXPECT_TRUE(send_int(fd, 0)); /* kept child files */