jsonl: NDJSON / JSON-Lines stream decoder

[OmarAlJarrah]

Summary

Adds a jsonl package that decodes a stream of JSON values (JSON Lines / NDJSON)
into typed values via a range-over-func iterator — for consuming streaming JSON
APIs.

What's included

• Decode[T any](r io.Reader) iter.Seq2[T, error] — yields each successive
  JSON value decoded into a T. Values may be separated by any JSON whitespace
  (newlines for NDJSON, or none), built on encoding/json's Decoder (incremental,
  no whole-stream buffering). Clean io.EOF ends iteration; a decode error
  (including a truncated final value) is surfaced once, then iteration stops.

Background

One of the streaming helpers deferred from the HTTP-value-types roadmap item. The
chunked-transfer side is already handled by net/http (the body is de-chunked
before Decode sees it).

Behavior changes

None — new package only.

Test plan

• go build ./...
• go vet ./...
• gofmt -l . (clean)
• go test -race ./... (all packages pass)
• Covered: NDJSON order; bare scalars; a single value without a trailing newline;
  empty stream → no values; a malformed value mid-stream surfaces the error after
  the first value; a truncated final value surfaces an error; early break stops
  decoding.

🤖 Generated with Claude Code