Language: silence @ on assignment RHS (#29132)

[PurHur]

Summary

• Closes Language: @$undef as assignment RHS still warns — Zend silences (ZEND_BEGIN_SILENCE) #29132: $a = @$undef; no longer emits Undefined variable on VM/JIT (Zend-silent).
• Root cause: php-cfg #13587 used UnaryPlus inside @ to force a CV check, but returned the original CV; post-END_SILENCE assign re-fetched it. UnaryPlus also coerces null→0, so it cannot be the @ expression value.
• Fix: materialize bare @$cv with Assign into a temp under silence (preserves null); return that result. Compiler treats Assign as an error-suppress primary for slot inheritance.

php-src: Zend/zend_compile.c / zend_execute.c (ZEND_BEGIN_SILENCE).

Test plan

• Issue repro (stderr empty, stdout ok) — VM + JIT
• vendor/bin/phpunit test/unit/ErrorControlOperatorTest.php — 3/3
• vendor/bin/phpunit --filter at_silence_assign_undef_rhs test/compliance/ErrorSuppressJITTest.php — OK
• #13587 error_get_last_at_read_stmt.phpt still OK on VM
• @$b = 1 / $x = @$c = 2 unchanged vs Zend

Verification transcript

$ php bin/vm.php /tmp/at_rhs.php 2>&1   # $a=@$undef; echo ok
ok
$ php bin/jit.php /tmp/at_rhs.php 2>&1 | grep -v Deprecated
ok
$ vendor/bin/phpunit test/unit/ErrorControlOperatorTest.php
OK (3 tests, 5 assertions)

Pre-triage: user_release_ready: yes (north-star5-fast ok); inventory doc drift pre-existing on master (unrelated).

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