Fix keyed list destruct Undefined array key warning line (#31994)

[PurHur]

Summary

• Stamp TYPE_ARRAY_DIM_FETCH / TYPE_ARRAY_DIM_FETCH_WRITE opcodes with CFG sourceLocation via assignSourceMetadata() so Undefined array key warnings cite the list-destructure fetch site (Zend ZEND_FETCH_LIST_R opline), not a prior statement such as echo.
• Root cause: without metadata, FatalSite::lineFromOpcodes() walked back to the preceding TYPE_ECHO for keyed destruct after an earlier echo; positional destruct accidentally worked because an intervening TYPE_ASSIGN on the same line was found first.
• Add compliance .phpt + dedicated VM/JIT test classes (Language: keyed list destructure Undefined array key Warning cites previous line — Zend list site (zend_vm_def.h) #31994).

php-src reference

• Zend/zend_vm_def.h — ZEND_FETCH_LIST_R
• Zend/zend_execute.c — zend_undefined_index uses current opline

Verification

Issue repro (VM):

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_list_undef_key_line.php 2>&1'

PHP Warning:  Undefined array key 1 in .../maintainer_gap_list_undef_key_line.php on line 3
a=1
PHP Warning:  Undefined array key "x" in .../maintainer_gap_list_undef_key_line.php on line 5
c_done

Issue repro (JIT — falls back to VM interpreter when MCJIT verify fails on this script):

./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/maintainer_gap_list_undef_key_line.php 2>&1'

Same output (key "x" now line 5, was line 4).

Compliance:

./script/phpunit.sh test/compliance/ListDestructUndefKeyLine31994VMTest.php
./script/phpunit.sh test/compliance/ListDestructUndefKeyLine31994JITTest.php

Both OK (1/1).

Not covered

• Full VMTest / JITTest suites
• AOT path (list destruct dim fetch uses same compiler metadata; no separate AOT repro run)
• Differential sweep (warning line only; no operand/lowering change)

Closes #31994

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