stream_select() ignores bytes already buffered in the stream

[NikitaSomik]

When several bytes arrive at once, fread(STDIN, 1) reads them all into the
stream's buffer. After that, stream_select() only checks the file
descriptor, so it reports "no data" and the remaining bytes are never read.
PHP counts buffered data as readable.

In a terminal this loses multi-byte keys: an arrow key (ESC [ A) shows up
as just ESC.

Reproduce

<?php
$got = '';
for ($i = 0; $i < 3; $i++) {
    $r = [STDIN]; $w = $e = null;
    if (stream_select($r, $w, $e, 0, 200_000) < 1) {
        break;
    }
    $got .= fread(STDIN, 1);
}
echo "read: '", $got, "'\n";

(printf abc; sleep 1) | phpr select.php

• PHP 8.5.7: read: 'abc'
• phpr: read: 'a'

Environment: macOS 26.6.2 (Apple M1 Pro), Rust 1.96.0, commit 352d4129.