Add local forwarding example

[blaubaer]

Reimplements gliderlabs/ssh#130 by @XSAM for the current standalone example structure.

Summary

• add a dedicated local TCP forwarding example with the required direct-tcpip channel handler
• restrict forwarding to the hard-coded loopback target 127.0.0.1:8080
• remove the ineffective local-forward callback from the remote-forward example
• let ordinary example sessions finish instead of blocking forever and correct the development startup message
• compile the new standalone module in the example CI matrix
• fix a pre-existing flaky streamlocal test assertion exposed by the PR race run

Testing

• go test -count=1 ./...
• go test -race -count=1 ./...
• affected streamlocal test with 100 normal and 20 race repetitions
• go vet and golangci-lint for the root and changed examples
• go test -mod=readonly for both changed example modules
• Windows amd64 and Darwin arm64 cross-builds for ssh-localforward
• go mod tidy -diff and git diff --check