fix(webserver): php_native_split function various issues - #146

Two small things worth fixing before merge:

1. Sign-compare warning — piece_count is int, split_limit is unsigned int, so piece_count >= split_limit - 1 will warn under -Wsign-compare in both branches. Making piece_count unsigned int cleans it up.

2. Limit convention — PHP's split($pattern, $string, $limit = -1) uses -1 as "no limit". The PR treats 0 as the default and only positive values as the cap, so a caller passing PHP's documented -1 would underflow to UINT_MAX (accidentally unlimited). Switching split_limit to signed int with -1 default and checking split_limit > 0 aligns with PHP semantics and avoids the underflow.

The four core fixes themselves all check out. Thanks for the contribution.

Drop the if ( split_limit <= 0 ) block — no return after it makes it inconsistent with the other error paths in this function, and -1 is the PHP-documented sentinel for "no limit" so emitting an error on it defeats the convention. The -1 default + the loop's split_limit > 0 gate already handle "no limit" cleanly.

si = get_scope_item(g_current_scope, "__param_2");
if ( si ) {
    PHP_VALUE_NODE *limit_node = &si->var->value;
    cast_value_dnum(limit_node);
    split_limit = limit_node->int_val;
}

No need to squash.

Now matches PHP semantics cleanly — -1 default, split_limit > 0 gate handles "no limit" silently, no spurious error. Thanks for the back-and-forth.