Non-spawning API for master and outstation creation

[jadamcrain]

Problem

The DNP3 library calls tokio::spawn internally in all spawn_* creation functions. This prevents callers from controlling which Tokio runtime or task set the futures are placed on.

Status

feature/no-spawn implements the master portion with unstable MasterBuilder / UdpMasterBuilder APIs. Associations are registered synchronously with add_association(...), which fails immediately on duplicate addresses and returns a scoped AssociationBuilder<'_> for adding polls. into_handle() releases the builder borrow, and the transport-specific into_* methods produce a MasterTask with async fn run(self).

The TCP outstation Server::into_task(listener) API synchronously consumes an already-bound listener and returns a ServerHandle plus an unpolled server task future. Existing stable bind_no_spawn() delegates to the same internal construction path.

The serial outstation API uses a fail-fast typestate flow: SerialOutstation::new(...) configures the protocol implementation synchronously, open(path, settings).await produces a SerialOutstationTask only after the port opens successfully, and run(self) permanently ends on shutdown or serial-port failure. The retrying spawn_outstation_serial_2() remains a separate stable API.

Remaining work

• Outstation serial: fail-fast SerialOutstation → open() → SerialOutstationTask::run() flow with no internal spawn or retry loop
• Outstation TCP server: Server::into_task(listener) takes an already-bound listener synchronously and returns the server handle and task future
• Gate the new master builder/task APIs behind an unstable feature
• Document the semver exemption beside the unstable feature in Cargo.toml
• Gate the new outstation task APIs behind unstable
• Compile and test the unstable feature in CI
• Ship in 1.7.0

unstable feature flag

Only the non-spawning APIs introduced as part of this 1.7.0 work go behind an unstable Cargo feature:

• Exempt from semver: while items remain behind the flag, they may change or disappear in any release, including patch releases. This policy is documented beside the feature in Cargo.toml; the gated items are absent from the default published rustdoc.
• No FFI schema entries and no guide pages for gated APIs.
• Covers: MasterBuilder, UdpMasterBuilder, AssociationBuilder, MasterTask, Server::into_task, SerialOutstation, and SerialOutstationTask.
• Existing stable APIs, including Server::bind_no_spawn and Server::add_outstation_no_spawn, are explicitly excluded and remain ungated.
• General connection-handler conveniences such as the EndpointList::into_connect_handler* methods are separate stable API work and are not covered by this gate.

#276 concerns the existing stable no_spawn methods and is outside the scope of this issue.

Non-goals

• FFI / language binding changes
• Additional TLS-specific API work
• Changes to existing public config types (semver)