php-in-php: pack('i'/'I') sizeof(int)=4 + backed enum parity (#5713)

[PurHur]

Summary

• Fix pack('i'/'I') and matching unpack() to use 4 bytes (sizeof(int) per php-src ext/standard/pack.c) instead of PHP_INT_SIZE (8 on 64-bit hosts).
• pack() VM builtin now runs operand validation even when the return value is discarded (Stdlib: VM builtins skip arg validation when return is discarded (returnVar null early-return) #5896 tail).
• Compliance PHPTs extended with pack('i', E::A) length check; fixed ?? coalesce in test bodies (parser gap).

C runtime: 0 lines added; all logic in PackEngine / JIT lowering.

Closes #5713

php-src reference

• ext/standard/pack.c — PHP_FUNCTION(pack), 'i'/'I' use sizeof(int)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/parity_pack_backed_enum.php
php bin/vm.php test/repro/parity_pack_backed_enum.php
vendor/bin/phpunit --filter PackEngineTest
vendor/bin/phpunit --filter pack_backed_enum
'

Output:

len=4   # Zend
len=4   # VM
OK (8 tests, 23 assertions)   # PackEngineTest
OK (3 tests, 3 assertions)    # pack_backed_enum VM + JIT

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