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_append.py

@@ -138,7 +138,7 @@ class TestAppend:
         self._place(source, REL, prefix + added)
         self._place(self._dest_file(source, dest, ""), REL, prefix)
 
-        result, _ = run_client(source, dest, flags=["--append", "-m"], port=shared_server.port)
+        result, _ = run_client(source, dest, flags=["--append", "--threads"], port=shared_server.port)
         assert result.returncode == 0, \
             f"--append -m failed: {(result.stderr or result.stdout)[:400]}"
         assert self._read(self._dest_file(source, dest, ""), REL) == prefix + added