Reject lone list spread assignment at compile time (#6936)

[PurHur]

Summary

• Reject lone [...$a] = $rhs list assignments at compile time with Zend-compatible CompileFatal message (Spread operator is not supported in assignments).
• Valid [$head, ...$tail] = $src destructuring from Language: list destructuring with spread — [$a, ...$rest] = $arr (zend_compile.c) #4835 remains unchanged.

php-src reference: Zend/zend_compile.c list assignment unpack validation.

PHP implementation: lib/Compiler.php — rejectLoneListSpreadAssign() called from compileListDestructGroup().

Closes #6936

Test plan

• Issue repro fatals on VM
• [$a, ...$rest] regression still prints 1:2,3,4
• Compliance .phpt added

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/maintainer_lone_list_spread_assign.php 2>&1 | head -1
# Fatal error: Spread operator is not supported in assignments in /compiler/test/repro/maintainer_lone_list_spread_assign.php on line 2

php bin/vm.php test/repro/maintainer_lone_list_spread_assign.php 2>&1 | head -1
# Fatal error: Spread operator is not supported in assignments in /compiler/test/repro/maintainer_lone_list_spread_assign.php on line 2

php bin/vm.php test/repro/list_destructuring_spread_4835.php
# 1:2,3,4

vendor/bin/phpunit --filter lone_list_spread
# OK (2 tests, 4 assertions)

vendor/bin/phpunit --filter list_destructuring_spread
# OK (2 tests, 2 assertions)

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