Register nextafter() on 8.4.0-dev forward profile (#15584) - #15591

Summary

• Change CompilerVersion::supportsNextafter() to advertisesBuiltinSince('8.4.0') so function_exists('nextafter') is true on the 8.4.0-dev forward profile (php-src 8.4 math.c).
• Fix VmMath::nextafter() delegating to the VM-registered builtin (incorrect float + recursion risk).
• Fix InternalStrictArg::requireFloat() JITVariable → Variable typo blocking JIT/AOT lowering.
• Align VmVarExportFloat with Zend var_export for IEEE edge floats (e.g. nextafter(1.0, 0.0)).

Closes #15584

Verification

./script/docker-exec.sh -- bash -lc '
  php bin/vm.php test/repro/maintainer_gap_nextafter_forward_profile.php
  ./vendor/bin/phpunit --filter nextafter
  ./vendor/bin/phpunit test/unit/CompilerVersionGateTest.php test/unit/CompilerVersionBuiltinAdvertisementTest.php
  php bin/compile.php -l test/repro/maintainer_gap_nextafter_forward_profile.php
'

Output:

ok
OK (12 tests, 18 assertions)   # nextafter VM/JIT compliance + unit
OK (59 tests, 75 assertions)   # version gate tests
# AOT lint: exit 0

Note: full AotTest::nextafter_basic link/run still hits harness LLVM segfault (same as wordwrap_width_coerce on this host); VM + JIT + compile-only probe are green.

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