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.

config/config_test.go

@@ -0,0 +1,56 @@
+package config
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestSourceConfigExpectedKinds(t *testing.T) {
+	tests := []struct {
+		name string
+		cfg  SourceConfig
+		want []string
+	}{
+		{
+			name: "plural kinds preferred",
+			cfg: SourceConfig{
+				Kinds: []string{" observation ", "forecast"},
+				Kind:  "alert",
+			},
+			want: []string{"observation", "forecast"},
+		},
+		{
+			name: "legacy singular fallback",
+			cfg: SourceConfig{
+				Kind: " alert ",
+			},
+			want: []string{"alert"},
+		},
+		{
+			name: "empty kinds",
+			cfg:  SourceConfig{},
+			want: nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.cfg.ExpectedKinds()
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Fatalf("ExpectedKinds() = %#v, want %#v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestSourceModeNormalize(t *testing.T) {
+	if got := SourceMode(" Poll ").Normalize(); got != SourceModePoll {
+		t.Fatalf("Normalize poll = %q, want %q", got, SourceModePoll)
+	}
+	if got := SourceMode("STREAM").Normalize(); got != SourceModeStream {
+		t.Fatalf("Normalize stream = %q, want %q", got, SourceModeStream)
+	}
+	if got := SourceMode("").Normalize(); got != SourceModeAuto {
+		t.Fatalf("Normalize auto = %q, want %q", got, SourceModeAuto)
+	}
+}