hard-links: -H/--hard-links preserves inode relationships

Source files sharing (st_dev,st_ino) are recreated as hard links on the
destination; only the first member's data crosses the wire (siblings ride a
payload-less STATUS_HARDLINK frame). Ordering requires the single-FIFO-writer
receiver + forced sequential scan (documented). link()-failure falls back to a
byte-identical local copy. Rejects -s/--append. PROTOCOL_VERSION 2.11.0->2.12.0.
Review fixes: delete the dead HardLinkRegistry (ordering holds by FIFO writer),
and --existing no longer aborts when the first member is absent but the sibling
exists (leaves the sibling in place).

src/client/usage.c

@@ -180,6 +180,7 @@ void print_usage(void) {
   printf("  --copy-links        Transform symlinks into referent files\n");
   printf("  --safe-links        Skip symlinks that point outside transfer tree\n");
   printf("  --copy-unsafe-links  Only transform unsafe symlinks into referent files\n");
+  printf("  -H, --hard-links    Preserve hard-link relationships across the transfer\n");
   printf("  -S, --sparse        Handle sparse files efficiently\n");
   printf("  --inplace           Update files in-place (no temp+rename)\n");
   printf(