php-in-php: delete GcCollectCyclesStandaloneLlvm — standalone AOT GC via PHP helpers (#18630)

[PurHur]

Summary

• Delete last *StandaloneLlvm GC file (GcCollectCyclesStandaloneLlvm.php, ~516 lines) and ~500 lines of dead LLVM registry/collect code in GcCollectCyclesRuntime.php.
• Route standalone AOT phpc_gc_register / phpc_gc_collect_cycles_impl through GcCollectCyclesRegistryJitHelper + GcCollectCyclesStandaloneJitHelper (shared native scan in GcCollectCyclesNativeScanJitHelper).
• Embed path unchanged semantically: GcCollectCyclesJitHelper::collectCyclesEmbed now delegates native scan to the shared helper.

Closes #18630.

C/LLVM shrink

• Deleted: lib/JIT/Builtin/GcCollectCyclesStandaloneLlvm.php (516 lines)
• Removed: standalone LLVM registry globals (phpc_gc_objects[], marked/inbound arrays) and LLVM monolith branches in GcCollectCyclesRuntime.php

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit --filter "GcCollectCyclesStandaloneLlvmShrinkTest|GcCollectCyclesRuntimeStandaloneTest|GcCollectCyclesCollectRuntimeShrinkTest|GcCollectCyclesRegistryRuntimeShrinkTest"'
# OK (12 tests, 78 assertions)

make bootstrap-selfhost-vm-driver-execute-probe
# bootstrap-selfhost-vm-driver-execute-probe: OK

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php script/check-selfhost-spine-coverage-sync.php'
# check-selfhost-spine-coverage-sync: OK (spine covers all 4775 inventory files)

Note: ./script/check-aot-build-smoke.sh tier-1 hello-world build is already red on master (LLVM verify __value__writeHashtable mismatch) — not introduced by this PR.

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