Language: emit Undefined array key Warning on dim ++/-- (#30078)

[PurHur]

Summary

• VM: emit Undefined array key E_WARNING on FETCH_DIM_W when the next opcode is pre/post inc/dec and the key is missing (Language: $a[missing]++ omits Undefined array key Warning — Zend warns then treats as null (zend_execute.c) #30078).
• JIT: branch on offsetIsSetDim before write dimFetch for inc/dec and call __compiler_undefined_array_key_warning_* when absent.
• Compliance guard: test/compliance/cases/language/dim_inc_undefined_array_key_warning.phpt.

Verification

Issue repro (VM + JIT, PHP_COMPILER_PROFILE=8.4):

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
export PHP_COMPILER_PROFILE=8.4
printf "%s\n" "<?php
\$x=[]; \$x[0]++; echo \"done\n\"; var_export(\$x); echo PHP_EOL;
\$f=false; \$f[0]++; var_export(\$f); echo PHP_EOL;
" >/tmp/dim_inc_undef.php
php -d error_reporting=E_ALL -d display_errors=1 bin/vm.php /tmp/dim_inc_undef.php
php -d error_reporting=E_ALL -d display_errors=1 bin/jit.php /tmp/dim_inc_undef.php
'

Output (both backends):

W:Undefined array key 0
done
array ( 0 => 1, )
W:Automatic conversion of false to array is deprecated
W:Undefined array key 0
array ( 0 => 1, )

Not covered

• Full VMTest/JITTest shards; bootstrap/self-host gates (language-only VM/JIT change).

Closes #30078

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