fix(parity): init delete_suppressed; gate deleted-path retention

- Initialize PipelineContextSender.delete_suppressed=false: an uninitialized
  true silently skipped the --delete keep-set manifest under -m/--threads,
  so destination extras were never removed.
- Allocate/install the receiver deleted-path observer only when
  report_deletes is set (--info=del / -i / --out-format under --delete), cap
  the retained list at MAX_MANIFEST_ENTRIES, and free already-created lists
  on the receiver-pipeline create failure path.
- Validate report_deletes/report_stats/report_dest_info on receive.
- Correct stale comments (config.h report_deletes, delete_plan.h deleted
  count, multiprocessing.h stats locking, utils.h observer placement).
- Tests: sender delete_suppressed init, report_deletes gating (unit), and
  -m/--delete default delete-after keep-set removal (integration).

src/shared/config.c

@@ -206,7 +206,8 @@ static bool validate_received_config(const Config* config) {
          valid_wire_bool(config->preserve_perms) && valid_wire_bool(config->preserve_times) &&
          valid_wire_bool(config->preserve_owner) && valid_wire_bool(config->preserve_group) &&
          valid_wire_bool(config->munge_links) && valid_wire_bool(config->keep_dirlinks) &&
-         valid_wire_bool(config->fake_super) &&
+         valid_wire_bool(config->fake_super) && valid_wire_bool(config->report_dest_info) &&
+         valid_wire_bool(config->report_stats) && valid_wire_bool(config->report_deletes) &&
          (!config->copy_as_set || (config->copy_as_uid >= 0 && config->copy_as_gid >= 0)) &&
          (!config->use_compression ||
           (config->compression_level >= 1 && config->compression_level <= 22)) &&