Add CsvStreamResponse for streaming large CSV exports

[dereuromark]

Summary

Proof-of-concept CsvStreamResponse companion to CsvView for memory-efficient streaming of large CSV exports. Built on the new Cake\Http\Response\AbstractStreamResponse base proposed upstream in cakephp/cakephp#19431.

Implements the design pivot agreed in issue #89: drop the originally sketched CsvStreamView (a View subclass that would have had to disable most of the View machinery to stream) in favor of a Response subclass that matches core's JsonStreamResponse shape.

Usage

public function export()
{
    $rows = $this->Articles->find()->disableBufferedResults();

    return new CsvStreamResponse($rows, [
        'header' => ['id', 'title', 'created'],
        'extract' => ['id', 'title', ['created', '%s']],
    ]);
}

One line in the controller, no viewBuilder() gymnastics. Symmetric with JsonStreamResponse from core.

What it inherits from AbstractStreamResponse

• CallbackStream wiring + body callback
• output() / outputAndFlush() / flushOutputBuffers() primitives
• flushEvery threshold counter
• X-Accel-Buffering: no header
• Content-Type with App.encoding-derived charset
• Log::error() shim (gated on cakephp/log being installed)
• PSR-7 immutability (withStreamOptions() / getStreamOptions())

What it owns

CSV wire format: header / footer rows, extract (Hash paths, sprintf formats, callables, EntityInterface::toArray()), BOM-on-first-row, optional sep={delimiter} line, delimiter/enclosure/escape, in-field newline replacement, eol between rows, dual dataEncoding → csvEncoding transcoding via iconv (with strict / ignore / transliterate modes) or mbstring fallback, and the excel shorthand that forces BOM + CRLF + UTF-8 in one toggle.

The row-formatting logic mirrors CsvView::_generateRow() and _transcode() so the streaming and non-streaming paths emit byte-identical output for the same config. The duplication is explicit on purpose for this PoC; extracting a shared row formatter is a clean follow-up once both PRs land.

Mid-stream errors — tear cleanly

A row that cannot be encoded (unrenderable extract path, strict-mode transcoding failure, fputcsv refusal) is logged via Log::error() and the stream tears: no further rows, no footer, truncated CSV on the wire. Trades a partial response for a valid-up-to-the-error file plus a server-side log entry that surfaces in Sentry / equivalent — exactly what jose suggested in the issue thread.

Composer change — temporary

The cakephp/cakephp constraint points at the feature-abstract-stream-response branch on the dereuromark/cakephp fork via a repositories entry. This is a draft PR scaffold — once cakephp/cakephp#19431 merges to 5.next the constraint flips to dev-5.next (or ^5.4 after 5.4 tags), the repositories entry is removed, and minimum-stability reverts.

To prevent lock in and allow users to use the packagist in general with older Cake versions, I recommend though:

• Document the CsvStreamResponse as 5.4+ feature and fail early if used otherwise.

Open decisions

1. Class location — CsvView\Http\Response\CsvStreamResponse. Plugin namespace is CsvView\ and HTTP responses elsewhere in Cake live under Http\Response\, so this fits both. Alternative: drop into src/Response/ to skip the deeper path.
2. Shared row formatter — current PR duplicates _generateRow() / _transcode() from CsvView. Follow-up could extract to a trait or a CsvRowFormatter class once the shape is reviewed. Out of scope here to keep the diff focused.
3. Flush threshold default — left at the base's 1 (flush every row, lowest latency). Easy to change to 50 / 100 if reviewers prefer fewer flush calls at the cost of slightly delayed first-byte.

Opening as draft — blocked on cakephp/cakephp#19431 review/merge, and on the three decisions above.