feat(sources)!: split source contracts into PollSource/StreamSource and add mode-aware source config

- Introduce explicit source interfaces: sources.PollSource and sources.StreamSource, with shared sources.Input (Name() only).
- Remove mandatory Kind() from the base source contract to support sources that emit multiple kinds.
- Add config.SourceMode (poll, stream, or omitted/auto) and SourceConfig.Kinds (plural expected kinds), while keeping legacy SourceConfig.Kind for compatibility.
- Enforce mode semantics in config validation (poll requires every, stream forbids every) and detect mode/driver mismatches in sources.Registry.
- Update docs and tests for the new source model and config behavior.

sources/source.go

@@ -7,34 +7,33 @@ import (
 )
 
 // Input is the common surface shared by all source types.
+//
+// A source may be polling (PollSource) or event-driven (StreamSource).
+// Both source types emit domain-agnostic event.Event values.
 type Input interface {
 	Name() string
-	Kind() event.Kind
 }
 
-// Source is a configured polling job that emits 0..N events per poll.
+// PollSource is a configured polling source that emits 0..N events per poll.
 //
-// Source implementations live in domain modules (weatherfeeder/newsfeeder/...)
+// PollSource implementations live in domain modules (weatherfeeder/newsfeeder/...)
 // and are registered into a feedkit sources.Registry.
 //
-// feedkit infrastructure treats Source as opaque; it just calls Poll()
+// feedkit infrastructure treats PollSource as opaque; it just calls Poll()
 // on the configured cadence and publishes the resulting events.
-type Source interface {
+type PollSource interface {
 	// Name is the configured source name (used for logs and included in emitted events).
 	Name() string
 
-	// Kind is the "primary kind" emitted by this source.
-	//
-	// This is mainly useful as a *safety check* (e.g. config says kind=forecast but
-	// driver emits observation). Some future sources may emit multiple kinds; if/when
-	// that happens, we can evolve this interface (e.g., make Kind optional, or remove it).
-	Kind() event.Kind
-
-	// Poll fetches from upstream and returns 0..N events.
+	// Poll fetches/processes one input batch and returns 0..N events.
+	// A single poll can emit multiple event kinds.
 	// Implementations should honor ctx.Done() for network calls and other I/O.
 	Poll(ctx context.Context) ([]event.Event, error)
 }
 
+// Source is a compatibility alias for the legacy polling-source name.
+type Source = PollSource
+
 // StreamSource is an event-driven source (NATS/RabbitMQ/MQTT/etc).
 //
 // Run should block, producing events into `out` until ctx is cancelled or a fatal error occurs.
@@ -43,3 +42,14 @@ type StreamSource interface {
 	Input
 	Run(ctx context.Context, out chan<- event.Event) error
 }
+
+// KindSource is an optional interface for sources that advertise one "primary" kind.
+// This is legacy-friendly but no longer required.
+type KindSource interface {
+	Kind() event.Kind
+}
+
+// KindsSource is an optional interface for sources that advertise multiple kinds.
+type KindsSource interface {
+	Kinds() []event.Kind
+}