Add non-spawning master and outstation task APIs

[jadamcrain]

Summary

• add unstable non-spawning master builders for TCP, TLS, serial, and UDP
• make association configuration scoped to its parent builder and return usable association and poll handles synchronously
• defer configured poll and keep-alive timers until the master task is first polled
• add unstable Server::into_task(listener) for TCP outstations with an already-bound listener
• add a fail-fast serial typestate flow: SerialOutstation::new → open → SerialOutstationTask::run
• gate only the new 1.7.0 APIs behind unstable and document their semver exemption
• compile and test the unstable feature in CI

Motivation

The existing spawning APIs choose where tasks run by calling tokio::spawn internally. These APIs let applications finish configuration and obtain concrete tasks or futures without spawning, so callers control the Tokio runtime and task set.

The type structure prevents associations from being attached to the wrong master and prevents a serial outstation task from existing until its port has opened successfully.

Existing stable APIs such as Server::bind_no_spawn and Server::add_outstation_no_spawn remain ungated. Stable examples continue to use the stable spawning API.

Behavior fixes

• initially enabled TCP and serial sessions no longer emit a transient Disabled state
• preconfigured poll and keep-alive periods begin when MasterTask::run is first polled rather than during configuration
• TCP server handles and tracing use the supplied listener's actual local address

Validation

• cargo test -p dnp3
• cargo test --features unstable
• stable and unstable Clippy with all targets and warnings denied
• stable, unstable, and minimal feature builds
• release build of dnp3, dnp3-ffi, and dnp3-ffi-java with minimal unstable features
• unstable rustdoc
• cargo fmt --all
• git diff --check

closes #414