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/client/usage.c

@@ -24,6 +24,16 @@ void print_usage(void) {
   printf("  -P                  Partial mode with progress (retention incomplete)\n");
   printf("  -8, --8-bit-output  Leave high-bit characters unescaped in output\n");
   printf("  --delete            Delete files on receiver not in source\n");
+  printf("                      (default timing: delete only after the whole\n");
+  printf("                      transfer has succeeded)\n");
+  printf("  --delete-before     Delete extras before the transfer starts\n");
+  printf("                      (implies --delete)\n");
+  printf("  --delete-during     Delete extras once the keep-set is known, before\n");
+  printf("  --del               data is applied (alias --del; implies --delete)\n");
+  printf("  --delete-delay      Delete extras only after a successful transfer\n");
+  printf("                      (implies --delete)\n");
+  printf("  --delete-after      Alias of the default --delete timing: delete only\n");
+  printf("                      after the transfer succeeded (implies --delete)\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, -d, --old-dirs, --old-d  Transfer the named directory entries without\n");
@@ -37,7 +47,6 @@ void print_usage(void) {
   printf("                    parent directory is not itself listed\n");
   printf("  --mkpath          Create the destination root directory on the server when it\n");
   printf("                    does not exist yet\n");
-  printf("  --del               Alias for --delete-during (not implemented)\n");
   printf("  --exclude <pattern> Exclude files matching pattern\n");
   printf("  --include <pattern> Only include files matching pattern\n");
   printf("  --exclude-from <file> Read exclude patterns from file\n");