fix(tailscale-router): add MASQUERADE to POSTROUTING, not nixos-nat-post

extraCommands runs after nixos-nat-post is deleted but before it is
re-created, so -A nixos-nat-post silently fails every time.  POSTROUTING
is a built-in chain that always exists; target it directly instead.

The -C idempotency check prevents duplicate rules on firewall reloads.
Drop networking.nat.enable -- it was only needed for the sub-chain that
turned out to be the wrong target.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/build-types/tailscale-router.nix

@@ -16,17 +16,27 @@
   # Must also be approved in the Tailscale admin console (Machines → Edit route settings).
   services.tailscale.extraUpFlags = [ "--advertise-routes=${vars.lanCidr}" ];
 
-  # Forwarded subnet-router traffic arrives on tailscale0 already
-  # tailscale-authenticated -- the firewall's normal per-port allow-list
-  # would otherwise drop it. Standard NixOS/Tailscale subnet-router guidance.
-  networking.firewall.trustedInterfaces = [ "tailscale0" ];
+  networking.firewall = {
+    # Forwarded subnet-router traffic arrives on tailscale0 already
+    # tailscale-authenticated -- the firewall's normal per-port allow-list
+    # would otherwise drop it. Standard NixOS/Tailscale subnet-router guidance.
+    trustedInterfaces = [ "tailscale0" ];
 
-  # SNAT traffic from LAN machines going out through Tailscale so the remote
-  # peer sees it sourced from this router's Tailscale IP (100.x.x.x) rather
-  # than a raw LAN IP.  Without this, Tailscale drops the forwarded packets
-  # because the source is not a recognised Tailscale address.
-  networking.nat = {
-    enable = true;
-    externalInterface = "tailscale0";
+    # SNAT LAN traffic going into Tailscale so the remote peer sees it as
+    # coming from this router's Tailscale IP rather than a raw LAN IP.
+    # Without this, Tailscale drops forwarded packets whose source is not a
+    # recognised Tailscale address.
+    #
+    # We target POSTROUTING directly (always-existing built-in chain) rather
+    # than nixos-nat-post: extraCommands runs after the old nixos-nat-post is
+    # deleted but before the new one is created, so -A nixos-nat-post silently
+    # fails.  The -C check makes the rule idempotent across firewall reloads.
+    extraCommands = ''
+      iptables -t nat -C POSTROUTING -s ${vars.lanCidr} -o tailscale0 -j MASQUERADE 2>/dev/null || \
+        iptables -t nat -A POSTROUTING -s ${vars.lanCidr} -o tailscale0 -j MASQUERADE
+    '';
+    extraStopCommands = ''
+      iptables -t nat -D POSTROUTING -s ${vars.lanCidr} -o tailscale0 -j MASQUERADE 2>/dev/null || true
+    '';
   };
 }