Restore the release trigger and ship 3.1.1

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Fixes a release I broke in #50, and ships the same work as 3.1.1.

What went wrong

#50 removed every automatic trigger from rust.yml, which correctly stopped
pull request runs and incorrectly stopped releases.

The publish job is gated on github.event_name == 'push' && github.ref == 'refs/heads/master' and reads github.event.before to diff Cargo.toml. Both
only exist on a push event. With workflow_dispatch alone that job could never
run, and a manual dispatch would not have helped: github.event.before is empty
outside a push, so the diff would be meaningless.

So 3.1.0 merged to master, no workflow fired at all, and crates.io is still on
3.0.3.

What this does

• 4fdefda puts the master push trigger back and leaves pull_request
  off. Pull requests still do not run CI; master does, because on this
  repository a push to master is the release. workflow_dispatch stays.
• 719f81a bumps to 3.1.1. The publish step diffs Cargo.toml against the
  previous master commit, so restoring the trigger on its own would find no
  change and release nothing. No source change; 3.1.1 carries exactly the nagoya
  socket work 3.1.0 was meant to carry.

3.1.0 will exist in this repository's history and never on crates.io. That is
the honest outcome and it costs a patch number.

Local gate

cargo build                                          ok
cargo test --no-default-features \
  --features ws-core,framed-transport,nagoya-transport   98 passed, 0 failed
cargo fmt --all -- --check                           clean
cargo clippy --no-default-features \
  --features ws-core,framed-transport,nagoya-transport -- -D warnings   clean

a_wire_message_crosses_a_nagoya_unix_socket_in_both_directions ... ok

The workflow was parsed rather than eyeballed, since a bad trigger is the defect
being fixed:

triggers: ["push", "workflow_dispatch"]
push branches: ["master"]
pull_request present: false
publish if: github.event_name == 'push' && github.ref == 'refs/heads/master'

The if now matches a trigger that exists.

After merge

The push to master fires the workflow, Cargo.toml differs from the previous
commit, and cargo publish runs. Watch for 3.1.1 on crates.io rather than
assuming it; the whole point of this PR is that a merge without a run is a
release that did not happen.