Add WolverineHeartbeat emission (CritterWatch#72)

[jeremydmiller]

Summary

Ships the emit side of node-level heartbeats so external monitors (e.g. CritterWatch) can detect when a Wolverine node has gone dark. Operators opt in with one line and route heartbeats wherever they want via the normal publish pipeline.

opts.EnableHeartbeats(TimeSpan.FromSeconds(10));
opts.PublishMessage<WolverineHeartbeat>().ToRabbitExchange(\"monitoring\");

What shipped

• HeartbeatPolicy on WolverineOptions.Heartbeat — Enabled (default true) and Interval (default 30s).
• WolverineHeartbeat record — ServiceName, NodeNumber, SentAt, Uptime.
• HeartbeatBackgroundService — BackgroundService that publishes on each interval tick using new MessageBus(runtime).PublishAsync(...). Bails out early when Enabled == false. Swallows-and-logs publish exceptions so a transport hiccup never crashes the host.
• EnableHeartbeats(TimeSpan?) extension — wires the singleton + hosted service registration; supplied interval (when given) overrides the policy.
• Docs page at docs/guide/runtime/heartbeats.md with a quickstart, field reference, and configuration notes; linked from the General nav.

What's out of scope

The consumption side — tracking received heartbeats per node, alerting on missing ones, and visualizing health — is intentionally CritterWatch's job and lives in that repo (CritterWatch#72).

Test coverage

Added src/Testing/CoreTests/Runtime/Heartbeat/HeartbeatBackgroundServiceTests.cs with five tests covering:

• Repeated publishes at the configured interval (50ms cadence over ~250ms ⇒ ≥2 publishes).
• Heartbeats carry ServiceName and NodeNumber from WolverineOptions/Durability.
• No publishes happen when HeartbeatPolicy.Enabled == false.
• EnableHeartbeats sets the policy and registers the hosted service in WolverineOptions.Services.
• EnableHeartbeats() without an interval preserves the default 30-second cadence.

dotnet test src/Testing/CoreTests/CoreTests.csproj is green on net8.0, net9.0, and net10.0 (1538 / 1542 / 1542 tests, 0 failures).

Companion work

This pairs with CW#73 (active-pull node health checks) and CW#70 (transport health probes) for a coherent monitoring story: heartbeats are the cheap passive signal, the other two cover "alive but degraded" cases.

Test plan

• dotnet build src/Wolverine/Wolverine.csproj — clean.
• dotnet test src/Testing/CoreTests/CoreTests.csproj — passing on all three target frameworks.
• Reviewer eyes on the publish-via-MessageBus approach and the placement of EnableHeartbeats (chose top-level WolverineOptionsExtensions.cs to match the plan; happy to relocate next to the heartbeat folder if preferred).

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