feat(delete): per-directory delete plans for --delete-during/--delete-delay (protocol 2.24.0)

Stream one delete plan per source directory from sender to receiver instead of
a single whole-tree keep-set manifest:

- --delete-during applies each directory's extras as its plan arrives, before
  that directory's data (rsync's generator-order deletion).
- --delete-delay snapshots each directory's extras while the plan arrives and
  commits the removals only after a fully-successful transfer, so files created
  after the scan survive (matching rsync's delete-delay, not delete-after).
- Type conflicts (a destination file blocking a source directory, or vice
  versa) are cleared immediately in both modes, so the nested write succeeds.

The plan carries the destination-relative directory, its kept child directory
names and its kept child file names; the first frame also carries the global
protected prefixes, size-skipped prefixes and --delete-missing-args paths.
--delete-before keeps the existing whole-tree early manifest; plain --delete and
--delete-after keep the end-of-transfer manifest commit.

Preserves the existing safety surface: protected/size-skipped prefixes and the
--delay-updates/basis skips are honored at any depth, deletion is scoped to the
synchronized directories (--files-from), MAX_SERVER_DELETE_COUNT and
--max-delete (partial + exit 25) are shared across plans, symlinks are never
followed, and paths are confined to the receive root.

src/server/receiver_pipeline.h

@@ -41,6 +41,11 @@ typedef struct PipelineContextReceiver {
      transfer truly succeeded.  NULL in the early delete modes (which delete at
      the manifest). */
   DeleteManifest* deferred_manifest;
+  /* Per-directory delete session for --delete-delay: receive_thread snapshots
+     each plan's extras as it arrives and hands the session here instead of
+     committing while the disk writer may still be draining; server.c commits it
+     after both threads joined.  NULL for every other timing. */
+  DeletePlanSession* deferred_plans;
   /* Set by server.c when the deferred delete commit hit the --max-delete
      budget; the terminal success frame then carries STATUS_DELETE_LIMIT
      (rsync exit 25) while the transfer itself still succeeds. */