Stdlib: forward-profile phantom function_exists gates (#16086)

[PurHur]

Summary

• Split callable registration (supports*, languageProfileVersion) from introspection advertisement (advertises*, stable VERSION) for forward-profile builtins.
• function_exists() / extension_loaded('bcmath') now match Zend 8.2 reference probes even when PHP_COMPILER_PROFILE=8.4 enables calling fpow, mb_str_pad, and bc*.
• PHP implementation: ext/standard/BuiltinIntrospectionPolicy.php (php-src: ext/standard/basic_functions.c function table).

Closes #16086

Verification

./script/docker-exec.sh -- bash -lc '
  export PHP_COMPILER_PROFILE=8.4
  php bin/vm.php test/repro/maintainer_gap_fpow_phantom_registration.php
  php bin/vm.php test/repro/maintainer_gap_mb_str_pad_phantom_function_exists.php
  php bin/vm.php test/repro/maintainer_gap_bcmath_phantom_registration.php
  php bin/vm.php test/compliance/cases/stdlib/forward_profile_phantom_introspection.phpt
'
# ok / ok / ok / ok + callable=1

source script/php-env.sh
vendor/bin/phpunit --filter ForwardProfilePhantomIntrospection test/unit/ForwardProfilePhantomIntrospectionTest.php
vendor/bin/phpunit --filter forward_profile_phantom_introspection test/compliance/VMTest.php
vendor/bin/phpunit --filter CompilerVersionBuiltinAdvertisement test/unit/CompilerVersionBuiltinAdvertisementTest.php
# OK (2+1+58 tests)

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