php-in-php: delete phpc_json_decode.c, LLVM json_decode via StringJsonDecodeJit (#6202)

[PurHur]

Summary

• Delete lib/AOT/runtime/phpc_json_decode.c (649 C lines removed from lib/AOT/Linker.php).
• Add lib/JIT/Builtin/StringJsonDecodeJit.php — LLVM bodies for __compiler_json_decode, __compiler_json_validate, __compiler_json_last_error, __compiler_json_last_error_msg (mirrors former C parser; php-src ref: ext/json/php_json.c).
• Wire via StringJsonDecode::ensureLinked() in Type/String_.php + JitJsonDecode.php (same pattern as StringUnserialize / php-in-php: unserialize() parser — delete phpc_unserialize.c, VmUnserialize PHP path for VM+JIT+AOT #5991).

C/runtime shrink

Removed	Lines
lib/AOT/runtime/phpc_json_decode.c	649

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/unit/JIT/StringJsonDecodeRuntimeStandaloneTest.php
vendor/bin/phpunit test/compliance/JITTest.php --filter json_decode_jit
vendor/bin/phpunit test/unit/JsonValidateBuiltinTest.php'

StringJsonDecodeRuntimeStandaloneTest.php  OK (1 test, 8 assertions)
JITTest.php --filter json_decode_jit         OK (1 test, 1 assertion)
JsonValidateBuiltinTest.php                  OK (5 tests, 6 assertions)

Note: bin/compile.php / AotTest json_decode_assoc segfault on this harness for all scripts including master echo ok (pre-existing env issue, exit 139). JIT path is green.

Closes #6202

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