Stdlib: memory_get_usage / memory_get_peak_usage (Zend allocator parity) #3134

Summary

Implement memory_get_usage() and memory_get_peak_usage() so scripts and tests can introspect allocator usage like Zend PHP.

php-src reference

• Zend/zend_alloc.c — zend_memory_usage(), zend_memory_peak_usage()
• ext/standard/basic_functions.c — PHP_FUNCTION(memory_get_usage), PHP_FUNCTION(memory_get_peak_usage)

Current behavior (PurHur)

Not registered in ext/standard/Module.php.

Repro

<?php
$before = memory_get_usage();
$buf = str_repeat('x', 10000);
$after = memory_get_usage();
$peak = memory_get_peak_usage();
echo ($after > $before) ? "grew\n" : "flat\n";
echo ($peak >= $after) ? "peak_ok\n" : "peak_bad\n";

Zend PHP 8.x: grew and peak_ok.

This compiler: undefined function.

Acceptance

• VM builtins returning int byte counts; real_usage parameter documented (Zend uses different semantics for emalloc vs malloc — match php-src when real_usage=true).
• Monotonic peak tracking across request/runtime lifetime.
• Compliance PHPT with ordering checks (after > before, peak >= after).
• Register in Module.php; update capability matrix.

Notes

• Initial implementation may use process-level approximations if full Zend heap accounting is not available; document any intentional subset in PHPT NOTES.
• Do not use internal_* permanent stubs.