feat(cloak): egress SYN-ACK mangler + T2/T3 probe-response synthesizer

In-decky-netns NFQUEUE rewriter (window/option-order/IP-ID) and raw-socket
synthesizer for nmap probes Linux drops but the target OS answers (T2/T3),
driven by os_fingerprint.OS_MANGLE. Packet-shaping logic is pure and unit-tested
offline; scapy/netfilterqueue import lazily in the runtime loops. Entry:
python -m decnet.cloak (run by the base container; CAP_NET_ADMIN).

decnet/cloak/__init__.py

@@ -0,0 +1,30 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""
+DECNET cloak — egress TCP/IP fingerprint masquerading for deckies.
+
+sysctls (decnet/os_fingerprint.py) own GLOBAL packet fields. The cloak owns the
+SYN-ACK *shape* and stack *behaviours* sysctl can't reach, so a decky reads as
+its claimed nmap_os under active fingerprinting (nmap -O):
+
+  - mangler   : NFQUEUE rewrite of egress SYN-ACK (window, TCP option order,
+                IP-ID generation) to match the MangleProfile for the slug.
+  - responder : raw-socket synthesis of replies to probes the Linux kernel
+                drops but the target OS answers (nmap T2/T3).
+
+Both run INSIDE the decky's network namespace (CAP_NET_ADMIN), launched by the
+base container — never a sidecar (that would double container count per decky).
+Driven by os_fingerprint.get_os_mangle(nmap_os); a slug with no profile is a
+no-op (the real Linux stack already approximates it).
+"""
+from __future__ import annotations
+
+from decnet.cloak.mangler import build_synack_options, next_ipid
+from decnet.cloak.responder import ProbeKind, build_reply_fields, classify_probe
+
+__all__ = [
+    "build_synack_options",
+    "next_ipid",
+    "classify_probe",
+    "build_reply_fields",
+    "ProbeKind",
+]