feat: implement rsync delete timing (--delete-before/--delete-during/--delete-delay/--delete-after)

Deletion timing is now real and selected by the four rsync flags plus the
plain --delete default. Wire protocol bumps to 2.8.0: two new config
booleans (delete_during, delete_delay) are serialized and validated, joining
the existing delete_before/delete_after.

- Early modes (--delete-before, --delete-during/--del): the sender pre-scans
  the whole tree (paths only), transmits the keep-set manifest BEFORE any
  file data, and the receiver removes extras and acks STATUS_OK; the sender
  only streams data after the deletion committed. Deletion is thus performed
  even if a later transfer phase fails (rsync delete-before/during are
  destructive by definition). FastSync streams in a single scan so it cannot
  interleave per-directory like rsync delete-during; --delete-during selects
  the same engine mode as --delete-before (documented divergence).
- Late/commit modes (plain --delete, --delete-after, --delete-delay): the
  manifest closes the data stream and deletion is committed only after
  STATUS_FINISHED proves the whole transfer succeeded, preserving FastSync's
  commit-style safety. --delete-delay converges with --delete-after because
  FastSync never snapshots the destination during data flow (documented).
- The STATUS_MANIFEST frame is now self-delimiting and position-independent.
  Single-threaded receivers delete before the success frame; the -m receiver
  hands the keep-set to server.c, which commits the deletion only after the
  disk writer thread has drained (fixes a delete-vs-in-flight-temp race).
- Every timing flag implies --delete; at most one timing flag is allowed.
- Each timing flag implies --delete, matching rsync; conflicts are rejected.

src/shared/file_receive.h

@@ -10,7 +10,14 @@
 File* file_receive(const Config* config, int file_descriptor);
 File* file_receive_directory(int file_descriptor);
 File* receive_incremental_check(int fd, const Config* config, bool* skipped);
-int receive_manifest(int fd, const Config* config, int* next_status);
+/* Read a delete-manifest frame: entry count then paths (self-delimiting; the
+   leading STATUS_MANIFEST code has been consumed).  Returns an owned path
+   ArrayList, or NULL after signalling STATUS_ERROR on a malformed frame. */
+ArrayList* receive_manifest_entries(int fd);
+/* Remove destination entries under config->receive_root_directory that are not
+   in `manifest` (bounded walk, staging-dir skip).  The caller decides WHEN to
+   run it based on the negotiated delete timing. */
+bool manifest_delete_extras(const Config* config, ArrayList* manifest);
 
 /* Outcome of a single file_save_to_disk operation.  The receiver needs to
    distinguish "written" from "skipped" so --remove-source-files can be told