Plugin host metadata API migration

v1.4.4 separates persistent plugin host metadata from each registration's initialization context. Rebuild Go embedding applications and plugin binaries. HTTP, Bolt, storage keys, payloads and storage versions are unchanged.

Previous call or literal	Current contract
manager.SetContext(SubsystemContext{...})	manager.SetContext(SubsystemHost{...})
manager.SetHostContext(SubsystemContext{...})	manager.SetHostContext(SubsystemHost{...})
nornicdb.LoadPluginsFromDir(dir, hostContext)	nornicdb.LoadPluginsFromDir(ctx, dir, hostMetadata)
heimdall.LoadHeimdallPluginsFromDir(dir, subsystemContext)	heimdall.LoadHeimdallPluginsFromDir(dir, hostMetadata)
SubsystemContext{Config: config, Context: ctx}	SubsystemContext{Context: ctx, SubsystemHost: SubsystemHost{Config: config}}

hostMetadata is a *heimdall.SubsystemHost for the NornicDB loader, and a heimdall.SubsystemHost value for the Heimdall loader. The NornicDB loader's explicit ctx controls registration admission. The standalone Heimdall loader retains its existing startup-owned registration behavior.

host := heimdall.SubsystemHost{Config: config, Lifetime: owner}
manager.SetContext(host)
err := nornicdb.LoadPluginsFromDir(ctx, pluginDir, &host)

Plugins still implement Initialize(heimdall.SubsystemContext) error. Its host fields remain available through field promotion (received.Config, for example), but keyed composite literals must name the embedded SubsystemHost. Its Context lasts only through that registration's initialization: caller cancellation, host stop, and registration completion cancel it. Later actions use their callback context. Host setters retain metadata without retaining a request context; admitted initialization still receives caller values and cancellation. Ordinary SetContext updates metadata without reopening closed admission; SetHostContext retains the existing new-host completion barrier.