php-in-php: delete phpc_unserialize.c, LLVM via StringUnserializeJit (#5991)

[PurHur]

Summary

• Delete lib/AOT/runtime/phpc_unserialize.c (~344 lines) and remove it from lib/AOT/Linker.php
• Add lib/JIT/Builtin/StringUnserializeJit.php — LLVM bodies for __compiler_unserialize and phpc_session_decode_payload (php-src: ext/standard/var_unserializer.c subset, Stdlib: unserialize() — VM + LLVM JIT/AOT #1175)
• StringUnserialize.php now delegates to StringUnserializeJit instead of compiling/linking C bitcode at JIT link time
• Hook StringUnserialize::implement in Type/String_.php alongside StringSerialize

C runtime shrink

Removed	Lines
lib/AOT/runtime/phpc_unserialize.c	344

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/JITTest.php --filter unserialize'
# OK (3 tests, 3 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/VMTest.php --filter unserialize'
# OK (3 tests, 3 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/unit/JIT/StringUnserializeRuntimeStandaloneTest.php --no-configuration'
# OK (1 test, 6 assertions)

Closes #5991

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