Stdlib: str_increment/str_decrement phantom function_exists on 8.2 reference (#16292)

[PurHur]

Summary

• Split supportsStrIncrement() (callable registration; forward PHP_COMPILER_PROFILE=8.3+ or stable 8.4+) from new advertisesStrIncrement() (function_exists / get_defined_functions gate).
• Wire str_increment / str_decrement through BuiltinIntrospectionPolicy so default 8.4.0-dev reference profile matches Zend 8.2 (absent from introspection).
• Add maintainer repro + update forward-profile compliance PHPT (--ENV-- PHP_COMPILER_PROFILE=8.3).

php-src reference: ext/standard/string.c — PHP_FUNCTION(str_increment) / PHP_FUNCTION(str_decrement) (PHP 8.3); introspection gate pattern from #11842 / #16086 (zend_thread_id, mb_str_pad).

Test plan

• Issue repro (VM):

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_str_increment_phantom_function_exists.php; echo exit:$?'
# ok / exit:0

• Phantom compliance:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "VMTest::testCases.*str_increment_phantom|VMTest::testCases.*php84_math_string_builtins_phantom"'
# OK (3 tests)

• Forward profile still advertises + calls:

./script/docker-exec.sh -- bash -lc 'PHP_COMPILER_PROFILE=8.3 php bin/vm.php test/compliance/cases/stdlib/forward_profile_builtins.phpt'
# jv=yes / si=yes / sd=yes / b

• Unit gates:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "CompilerVersionGateTest::testSupportsStrIncrement|CompilerVersionGateTest::testAdvertisesStrIncrement|ForwardProfilePhantomIntrospectionTest::testStrIncrement"'
# OK (24 tests in str_increment filter set)

Closes #16292

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