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.

transport/http.go

@@ -14,7 +14,7 @@ import (
 // or malicious large responses.
 const maxResponseBodyBytes = 2 << 21 // 4 MiB
 
-// DefaultHTTPTimeout is the standard timeout used by weatherfeeder HTTP sources.
+// DefaultHTTPTimeout is the standard timeout used by HTTP sources.
 // Individual drivers may override this if they have a specific need.
 const DefaultHTTPTimeout = 10 * time.Second