[Bug] Numeric-string indexes are rejected for string offset access

[nahime0]

Description

PHP accepts numeric-string indexes for string offset access (for example "0" and "3"), coercing them the same way it does integer offsets. elephc rejects the same code at compile time with String index must be integer.

This is a PHP compatibility issue.

Reproduction

<?php
$s = "abc";
echo $s['0'];
echo "\n";
echo isset($s['3']) ? "y\n" : "n\n";

PHP output

a
n

elephc behavior

Compilation fails with:

error[3:8]: String index must be integer
error[5:14]: String index must be integer

Expected behavior

The program should compile and behave like PHP:

• $s['0'] should produce "a"
• isset($s['3']) should produce false

Actual behavior

elephc rejects numeric-string offsets instead of accepting PHP's coercion rules for string indexes.

Environment

• elephc 0.21.14
• PHP 8.4.20
• macOS ARM64

Notes

This looks like a type-checker restriction on string-offset indexing rather than a runtime issue.