Add veth capability probe + --no-veth network flag

Prep work for a tap+userspace-relay fallback: the real target device
supports tun/tap but not veth (CONFIG_VETH stripped from its kernel),
so -n/--network joins there can't use the veth-pair mechanism as-is.

probe_veth_support() (network_bridge.{h,cpp}) detects kernel veth
support the same way bwrap.cpp's kernel_supports_namespace() probes
namespace types: fork, unshare(CLONE_NEWNET) into a throwaway
namespace, try `ip link add ... type veth ...` there. should_use_veth()
combines that with a new per-network NetworkEntry::veth policy flag
(default true, config_file.h), mirroring how namespace_policy_enabled()
already combines kernel capability with policy for --unshare-xxx.

--no-veth at network-creation time (-n <name> --extern|--intern
--no-veth) sets veth: false, forcing the not-yet-built tap+relay
fallback even on a veth-capable kernel like this dev machine -- lets
that path be exercised here without the actual veth-less hardware.

Verified as root via the doas rule: probe_veth_support() returns true
on this dev machine (a real veth pair is created successfully), and
--no-veth correctly persists veth: false while should_use_veth() still
returns false regardless of kernel support.

The fallback itself (network_tap_relay.{h,cpp}) isn't wired in yet --
a --no-veth network simply has no way to join a container until that
lands.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

README.md

@@ -92,7 +92,7 @@ slocker-lite -V|--version
 | `--list-volumes` | List all named volumes (see `-v/--volume`) with their host directory. |
 | `--delete-volume <name>` | Remove a named volume from the config. The host directory is left untouched. |
 | `--delete-volume-full <name>` | Like `--delete-volume`, but also recursively deletes the volume's host directory. |
-| `-n, --network <name>` | Create/manage a persistent named network: requires exactly one of `--extern` (a real Linux bridge in the host's own namespace, with NAT/forwarding set up so containers on it reach the host's real network) or `--intern` (a bridge inside its own dedicated, routeless namespace, only reachable by other containers on the same network). `--subnet <cidr>` overrides the auto-allocated IPv4 range (`10.168.0.0/24`, incrementing per network); `--no-ipv6` disables (and `--subnet6 <cidr>` overrides) the auto-allocated IPv6 range, on by default. With `--run`, instead joins `<name>` to the container as its own `eth<N>` interface with an address from the network's subnet; repeatable, no membership limit. Root-only for now. See [`docs/networking-design.md`](docs/networking-design.md). |
+| `-n, --network <name>` | Create/manage a persistent named network: requires exactly one of `--extern` (a real Linux bridge in the host's own namespace, with NAT/forwarding set up so containers on it reach the host's real network) or `--intern` (a bridge inside its own dedicated, routeless namespace, only reachable by other containers on the same network). `--subnet <cidr>` overrides the auto-allocated IPv4 range (`10.168.0.0/24`, incrementing per network); `--no-ipv6` disables (and `--subnet6 <cidr>` overrides) the auto-allocated IPv6 range, on by default. `--no-veth` forces the tap+relay join fallback even on a kernel that supports veth (useful for testing that path; it's otherwise chosen automatically whenever the running kernel lacks veth support). With `--run`, instead joins `<name>` to the container as its own `eth<N>` interface with an address from the network's subnet; repeatable, no membership limit. Root-only for now. See [`docs/networking-design.md`](docs/networking-design.md). |
 | `--list-networks` | List all named networks (see `-n/--network`) with their kind, IPv4 subnet, and IPv6 subnet (or `(no ipv6)`). |
 | `--delete-network <name>` | Remove a named network from the config. |
 | `-p, --port-forward [<network>:]<host-port>:<container-port>` | With `--run`, forward a TCP port from the host into the container. `<network>` is optional, defaulting to the container's sole `--extern` network (an error if it joined more than one without specifying). Repeatable. Reachable via the host's real, externally-facing IP; `localhost`/loopback access has a known NAT-hairpinning limitation (see [`docs/networking-design.md`](docs/networking-design.md)). |
@@ -186,6 +186,7 @@ networks:
     subnet: 10.168.0.0/24
     ipv6: true
     subnet6: fd00:168:0:0::/64
+    veth: true
 ```
 
 `global.log-level` sets the default log verbosity (an explicit `--log-level` on