Stdlib: get_defined_constants(true) extension groups (#17416)

[PurHur]

Summary

• Add ExtensionConstantGroups registry so get_defined_constants(true) buckets VM-defined extension constants by extension name (calendar, hash, dom, filter, standard, etc.) instead of lumping them into user.
• Extract stdlib bootstrap constant maps into StdlibModuleConstants (single source for Module::init() and categorization).
• Add compliance guard for calendar / CAL_GREGORIAN; sync spine (+4 units).

php-src reference

• Zend/zend_builtin_functions.c — get_defined_constants() categorized buckets

PHP implementation

• ext/standard/ExtensionConstantGroups.php — extension name → registered constant map
• ext/standard/VmConstants.php — buildCategorized() iterates registry
• ext/standard/StdlibModuleConstants.php — stdlib/date/zlib/json bootstrap maps

Verification

./script/docker-exec.sh -- bash -lc \
  'echo "zend:"; php -r '\''$c=get_defined_constants(true); var_dump(defined("CAL_GREGORIAN")); var_dump(isset($c["calendar"]["CAL_GREGORIAN"]));'\''; \
   echo "vm:"; php bin/vm.php -r '\''$c=get_defined_constants(true); var_dump(defined("CAL_GREGORIAN")); var_dump(isset($c["calendar"]["CAL_GREGORIAN"]));'\'''
# zend: bool(true) bool(true)
# vm:  bool(true) bool(true)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "GetDefinedConstants|filter_defined_constants"'
# OK (13 tests, 20 assertions)

php script/check-selfhost-spine-coverage-sync.php
# OK (spine covers all 4278 inventory files)

Closes #17416

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