feat(os_fingerprint): Phase 1 — extend OS sysctls with 6 new fingerprint knobs

Add tcp_timestamps, tcp_window_scaling, tcp_sack, tcp_ecn, ip_no_pmtu_disc,
and tcp_fin_timeout to every OS profile in OS_SYSCTLS.

All 6 are network-namespace-scoped and safe to set per-container without
--privileged. They directly influence nmap's OPS, WIN, ECN, and T2-T6
probe groups, making OS family detection significantly more convincing.

Key changes:
- tcp_timestamps=0 for windows/embedded/cisco (strongest Windows discriminator)
- tcp_ecn=2 for linux (ECN offer), 0 for all others
- tcp_sack=0 / tcp_window_scaling=0 for embedded/cisco
- ip_no_pmtu_disc=1 for embedded/cisco (DF bit ICMP behaviour)
- Expose _REQUIRED_SYSCTLS frozenset for completeness assertions

Tests: 88 new test cases across all OS families and composer integration.
Total suite: 812 passed.

decnet/os_fingerprint.py

@@ -5,17 +5,27 @@ Maps an nmap OS family slug to a dict of Linux kernel sysctls that, when applied
 to a container's network namespace, make its TCP/IP stack behaviour resemble the
 claimed OS as closely as possible within the Linux kernel's constraints.
 
+All sysctls listed here are network-namespace-scoped and safe to set per-container
+without --privileged (beyond the NET_ADMIN capability already granted).
+
 Primary discriminator leveraged by nmap: net.ipv4.ip_default_ttl (TTL)
   Linux              → 64
   Windows            → 128
   BSD (FreeBSD/macOS)→ 64  (different TCP options, but same TTL as Linux)
   Embedded / network → 255
 
-Secondary tuning (TCP behaviour):
-  net.ipv4.tcp_syn_retries  – SYN retransmits before giving up
+Secondary discriminators (nmap OPS / WIN / ECN / T2–T6 probe groups):
+  net.ipv4.tcp_syn_retries    – SYN retransmits before giving up
+  net.ipv4.tcp_timestamps     – TCP timestamp option (OPS probes); Windows = off
+  net.ipv4.tcp_window_scaling – Window scale option; embedded/Cisco typically off
+  net.ipv4.tcp_sack           – Selective ACK option; absent on most embedded stacks
+  net.ipv4.tcp_ecn            – ECN negotiation; Linux offers (2), Windows off (0)
+  net.ipv4.ip_no_pmtu_disc    – DF bit in ICMP replies (IE probes); embedded on
+  net.ipv4.tcp_fin_timeout    – FIN_WAIT_2 seconds (T2–T6 timing); Windows shorter
 
 Note: net.core.rmem_default is a global (non-namespaced) sysctl and cannot be
-set per-container without --privileged; it is intentionally excluded.
+set per-container without --privileged; TCP window size mangling is deferred to
+Phase 2 (iptables entrypoint).
 """
 
 from __future__ import annotations
@@ -24,27 +34,59 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
     "linux": {
         "net.ipv4.ip_default_ttl": "64",
         "net.ipv4.tcp_syn_retries": "6",
+        "net.ipv4.tcp_timestamps": "1",
+        "net.ipv4.tcp_window_scaling": "1",
+        "net.ipv4.tcp_sack": "1",
+        "net.ipv4.tcp_ecn": "2",
+        "net.ipv4.ip_no_pmtu_disc": "0",
+        "net.ipv4.tcp_fin_timeout": "60",
     },
     "windows": {
         "net.ipv4.ip_default_ttl": "128",
         "net.ipv4.tcp_syn_retries": "2",
+        "net.ipv4.tcp_timestamps": "0",
+        "net.ipv4.tcp_window_scaling": "1",
+        "net.ipv4.tcp_sack": "1",
+        "net.ipv4.tcp_ecn": "0",
+        "net.ipv4.ip_no_pmtu_disc": "0",
+        "net.ipv4.tcp_fin_timeout": "30",
     },
     "bsd": {
         "net.ipv4.ip_default_ttl": "64",
         "net.ipv4.tcp_syn_retries": "6",
+        "net.ipv4.tcp_timestamps": "1",
+        "net.ipv4.tcp_window_scaling": "1",
+        "net.ipv4.tcp_sack": "1",
+        "net.ipv4.tcp_ecn": "0",
+        "net.ipv4.ip_no_pmtu_disc": "0",
+        "net.ipv4.tcp_fin_timeout": "60",
     },
     "embedded": {
         "net.ipv4.ip_default_ttl": "255",
         "net.ipv4.tcp_syn_retries": "3",
+        "net.ipv4.tcp_timestamps": "0",
+        "net.ipv4.tcp_window_scaling": "0",
+        "net.ipv4.tcp_sack": "0",
+        "net.ipv4.tcp_ecn": "0",
+        "net.ipv4.ip_no_pmtu_disc": "1",
+        "net.ipv4.tcp_fin_timeout": "15",
     },
     "cisco": {
         "net.ipv4.ip_default_ttl": "255",
         "net.ipv4.tcp_syn_retries": "2",
+        "net.ipv4.tcp_timestamps": "0",
+        "net.ipv4.tcp_window_scaling": "0",
+        "net.ipv4.tcp_sack": "0",
+        "net.ipv4.tcp_ecn": "0",
+        "net.ipv4.ip_no_pmtu_disc": "1",
+        "net.ipv4.tcp_fin_timeout": "15",
     },
 }
 
 _DEFAULT_OS = "linux"
 
+_REQUIRED_SYSCTLS: frozenset[str] = frozenset(OS_SYSCTLS["linux"].keys())
+
 
 def get_os_sysctls(nmap_os: str) -> dict[str, str]:
     """Return the sysctl dict for *nmap_os*.  Falls back to Linux on unknown slugs."""
@@ -54,3 +96,4 @@ def get_os_sysctls(nmap_os: str) -> dict[str, str]:
 def all_os_families() -> list[str]:
     """Return all registered nmap OS family slugs."""
     return list(OS_SYSCTLS.keys())
+