Add a base stream transport

The existing example is module/transport/base/tcp. A new base stream transport, say unix for Unix domain sockets, touches these files:

File	Change
module/transport/base/unix/settings.go (new)	Settings struct with JSON tags, Validate, Warnings
module/transport/base/unix/transport.go (new)	Transport type, Info, NewStreamDialer, NewStreamListener, and NewBaseStreamTransportModule
module/transport/base/unix/unix_test.go (new)	tests that run without core
core/registry/default.go	one line: moduleRegistry.Transports().RegisterBaseStream(unix.NewBaseStreamTransportModule)

Steps:

1. Settings. Declare the options as a struct. Defaults are whatever your constructor sets; Validate refuses impossible values at startup; Warnings reports risky but legal ones. Use config/types.Duration for durations and types.Secret for anything secret.
2. Kind. Implement spec.BaseStreamTransportModule: embed the settings (type Transport struct{ Settings }), return a fixed spec.ModuleInfo from Info (the registry checks it never depends on a setting), and build a spec.StreamDialer and a spec.StreamListener.
3. Streams. Returned streams must satisfy spec.Stream. If yours cannot half-close, CloseWrite returns spec.ErrNoHalfClose.
4. Resolution. A base transport is the only layer allowed to resolve a domain Destination. Do not resolve earlier, and do not resolve anything but your own target.
5. Binding. NewStreamListener binds immediately, so a port conflict fails at startup, and it refuses a zero bind address rather than listening on every interface.
6. Constructor. Export func NewBaseStreamTransportModule() spec.BaseStreamTransportModule. The return type is the kind, so registering it as the wrong kind does not compile.
7. Register. Add the one line to registry.Default(). The name from Info is now valid as a "type" in config files.