fix(landjail): allow TCP bind under Landlock

[openmarmot]

fix(landjail): allow TCP bind under Landlock

Problem

With --jail-type landjail, processes that listen on a local TCP port fail immediately (e.g. opencode web → ServeError / bind permission denied).

Landlock ABI V4 covers both bind_tcp and connect_tcp. Landjail used landlock.V4.RestrictNet(...) with only ConnectTCP(proxyPort) rules. Because bind was handled but never allowed, all binds were denied (EACCES), while connect to the proxy still worked.

Change

In landjail/child.go, restrict connect only:

• Handled rights: AccessNetConnectTCP
• Allowed: connect to the Boundary proxy port
• Bind/listen: unrestricted (not handled by Landlock)

Egress remains proxy-only. Local servers inside the jail can bind normally.

Docs updated in docs/architecture.md.

How to test

make build

# 1) Bind was broken before; should succeed now
./boundary --jail-type landjail --proxy-port 18080 --allow "domain=*" -- \
  python3 -c 'import socket; s=socket.socket(); s.bind(("127.0.0.1", 4096)); print("BIND_OK")'

# 2) Direct egress still blocked
./boundary --jail-type landjail --proxy-port 18080 --allow "domain=*" -- \
  python3 -c 'import socket; s=socket.socket(); s.connect(("1.1.1.1", 443))'
# expect: PermissionError / errno 13

# 3) Real workload that needs a listen port
./boundary --jail-type landjail --proxy-port 18080 --allow "domain=*" -- \
  opencode web --port 4096
# expect: opencode web works