# fix: honor start="0" on ordered lists

## Problem

Converting an ordered list whose `start` attribute is `0` produces the wrong
numbering: the list restarts at `1` instead of `0`.

```php
$converter = new League\HTMLToMarkdown\HtmlConverter();
$converter->convert('<ol start="0"><li>Item A</li><li>Item B</li></ol>');
// actual:   "1. Item A\n2. Item B"
// expected: "0. Item A\n1. Item B"
```

`<ol start="0">` is valid HTML, and CommonMark supports ordered lists that begin
at `0`, so the resulting Markdown is incorrect. Any other start value (for
example `start="120"`) works.

## Root cause

`src/Converter/ListItemConverter.php:55`. The start value was read inside a
truthiness check:

```php
if ($listType === 'ol' && ($parent = $element->getParent()) && ($start = \intval($parent->getAttribute('start')))) {
    $number = $start + $element->getSiblingPosition() - 1;
} else {
    $number = $element->getSiblingPosition();
}
```

`\intval('0')` is `0`, which is falsy, so an explicit `start="0"` takes the
`else` branch and the list is renumbered from the sibling position (1-based).
The check cannot distinguish "start attribute absent" from "start attribute set
to 0", because `getAttribute('start')` returns `''` when the attribute is missing
and `intval('')` is also `0`.

## The fix

Detect an explicitly provided numeric `start` attribute with `\is_numeric()`
instead of relying on the truthiness of `intval()`:

```php
if ($listType === 'ol' && ($parent = $element->getParent()) && \is_numeric($start = $parent->getAttribute('start'))) {
    $number = (int) $start + $element->getSiblingPosition() - 1;
} else {
    $number = $element->getSiblingPosition();
}
```

`\is_numeric('')` is `false` (missing attribute -> normal numbering) while
`\is_numeric('0')` is `true`, so `start="0"` is now respected. Existing values
such as `start="120"` are unaffected.

## Test

Added an assertion in `HtmlConverterTest::testLists`:

```php
$this->assertHtmlGivesMarkdown('<ol start="0"><li>Item A</li><li>Item B</li></ol>', "0. Item A\n1. Item B");
```

## Gate

```
composer install && vendor/bin/phpunit --no-coverage
```

Result: exit code `0` - `OK (50 tests, 230 assertions)`.

Counterfactual: reverting `ListItemConverter` to the `intval()` truthiness check
fails the new assertion - `<ol start="0">` yields `1. Item A\n2. Item B` instead
of `0. Item A\n1. Item B`.