feat(p7-cli-namespace): rename colliding short flags to rsync parity (Wave A)

-c -> --checksum, -m -> --prune-empty-dirs, -M -> --remote-option,
-f -> --filter, -s -> --secluded-args, -p -> --perms, -T -> --temp-dir;
-a/--archive is now real rsync -rlptgoD (links+metadata+devices+specials).

FastSync's own flags moved to long-form-only or new shorts:
-j/--threads (multithreading), --preserve (metadata), --sendfile,
--chunk-serialization, --timeout, --ssh-port. Client-side only; the
wire config fields are unchanged (no PROTOCOL_VERSION bump). The server
keeps -p as its port. Docs (README, RSYNC_COMPAT summary 129->132) and
unit/integration tests updated. 37/37 unit, 400-pass integration.

tests/integration/test_stop.py

@@ -231,7 +231,7 @@ class TestStopDelete:
         """-m immediate stop + --delete: the completion tail must not read the
         still-appendable manifest (no race) and must not delete the mirrors."""
         source, dest = self._seed("del_mt", shared_server.port, many=True)
-        result, _ = run_client(source, dest, flags=["-m", "--delete", "--stop-at=now+0s"],
+        result, _ = run_client(source, dest, flags=["--threads", "--delete", "--stop-at=now+0s"],
                                port=shared_server.port)
         assert result.returncode == 0, \
             f"-m --delete immediate stop failed: {(result.stderr or result.stdout)[:400]}"