// Package sources defines feedkit's input-source abstraction.
//
// A source ingests upstream input and emits one or more event.Event values.
//
// feedkit supports two source modes:
//   - PollSource: scheduler invokes Poll on a cadence.
//   - StreamSource: source runs continuously and pushes events as input arrives.
//
// Source drivers are domain-specific and registered into Registry by driver name.
// Registry can then build configured sources from config.SourceConfig.
//
// A single source may emit 0..N events per poll or stream iteration, and those
// events may span multiple event kinds.
//
// HTTP-backed polling sources can share NewHTTPSource for generic HTTP config
// parsing and conditional GET behavior. The helper understands:
//   - params.url
//   - params.user_agent
//   - params.conditional (optional, default true)
//
// When validators are available, NewHTTPSource prefers ETag/If-None-Match and
// falls back to Last-Modified/If-Modified-Since. A 304 Not Modified response is
// treated as a successful unchanged poll.
package sources