Remove os.Exit callsites below the entrypoint: phase 3 (plans/exit.md) (#2204)

Phase 3 of plans/exit.md: the streaming-interface change, the load-bearing
piece for #341 (DSL exit statement) and #440 (strict mode).

- RecordTransformer.Transform and RecordTransformerFunc now return error.
  All 69 Transform implementations and their dispatch helpers updated
  (mechanical rewrite, compiler- and errcheck-verified).
- runSingleTransformerBatch, on a Transform error, forwards any output
  produced before the failure plus an end-of-stream marker downstream, so
  the rest of the chain and the record-writer drain and finish cleanly;
  runSingleTransformer then surfaces the error to stream.Stream's select
  loop (non-blocking send; first error wins) and signals upstream-done so
  the record-reader stops. This is exactly the flush-then-exit sequencing a
  future DSL 'exit N' needs.
- dataProcessingErrorChannel and FileOutputHandler.recordErroredChannel are
  now chan error instead of chan bool; ChannelWriter still prints write-
  error details at the site and sends the 'exiting due to data error'
  sentinel, preserving the exact stderr shape pinned by regression cases.
- Mid-stream os.Exit sites converted to returned errors: put/filter DSL
  begin/main/end-block errors and the non-boolean filter-expression case,
  tee write/close failures, split write/open/close failures, join left-file
  ingest failures (both half-streaming and sorted paths, with full error
  plumbing through JoinBucketKeeper), histogram/stats2 ingest errors, surv
  fit errors, and step stepper allocation (tStepperAllocator now returns
  (tStepper, error); bad EWMA coefficients propagate; negative slwin
  parameters are reported by the CLI parser via the existing
  bad-stepper-name pattern).
- The two genuinely internal join-bucket-keeper states now use
  lib.InternalCodingErrorWithMessageIf instead of hand-rolled print+exit.
- pkg/transformers is now os.Exit-free.

Behavior notes: the non-boolean filter message gains the standard 'mlr: '
prefix and a newline (it previously printed with neither); tee errors now
include the underlying cause. All 4779 regression cases pass unchanged;
mlr head early-out latency is unaffected (0.02s over 50M records).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

pkg/stream/stream.go

@@ -2,7 +2,6 @@ package stream
 
 import (
 	"bufio"
-	"errors"
 	"io"
 
 	"github.com/johnkerl/miller/v6/pkg/cli"
@@ -66,10 +65,14 @@ func Stream(
 
 	// We're done when a fatal error is registered on input (file not found,
 	// etc) or when the record-writer has written all its output. We use
-	// channels to communicate both of these conditions.
+	// channels to communicate both of these conditions. The
+	// dataProcessingErrorChannel carries mid-stream errors from the
+	// transformer chain (e.g. DSL runtime errors, tee/split write failures)
+	// and from the record-writer; senders use a non-blocking send, so the
+	// first error wins and is returned once the writer finishes draining.
 	inputErrorChannel := make(chan error, 1)
 	doneWritingChannel := make(chan bool, 1)
-	dataProcessingErrorChannel := make(chan bool, 1)
+	dataProcessingErrorChannel := make(chan error, 1)
 
 	// For mlr head, so a transformer can communicate it will disregard all
 	// further input.  It writes this back upstream, and that is passed back to
@@ -84,7 +87,7 @@ func Stream(
 
 	go recordReader.Read(fileNames, *initialContext, readerChannel, inputErrorChannel, readerDownstreamDoneChannel)
 	go transformers.ChainTransformer(readerChannel, readerDownstreamDoneChannel, recordTransformers,
-		writerChannel, options)
+		writerChannel, dataProcessingErrorChannel, options)
 	go output.ChannelWriter(writerChannel, recordWriter, &options.WriterOptions, doneWritingChannel,
 		dataProcessingErrorChannel, bufferedOutputStream, outputIsStdout)
 
@@ -94,8 +97,8 @@ func Stream(
 		select {
 		case ierr := <-inputErrorChannel:
 			retval = ierr
-		case <-dataProcessingErrorChannel:
-			retval = errors.New("exiting due to data error") // details already printed
+		case derr := <-dataProcessingErrorChannel:
+			retval = derr
 		case <-doneWritingChannel:
 			done = true
 		}