Stdlib: withhold brotli on 8.2 reference profile (#17563)

[PurHur]

Summary

• Add BrotliExtensionPolicy + CompilerVersion::supportsBrotli() mirroring the ext/bz2 phantom gate (Regression: extension_loaded('bz2') / bzcompress() true — phantom API without ext/bz2 (ext/bz2) #11992).
• On the default 8.2 reference profile, extension_loaded('brotli') and function_exists('brotli_*') are false; forward profile (PHP_COMPILER_PROFILE=8.4) restores round-trip when libbrotli FFI is available.
• php-src reference: ext/brotli/brotli.c — optional extension absent unless built/loaded.

Test plan

• Issue repro (reference profile):

./script/docker-exec.sh -- bash -lc 'export PHP_COMPILER_PROFILE=8.2
php bin/vm.php test/repro/maintainer_gap_brotli_phantom_extension.php'
# ok

• Compliance phantom guard:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter brotli_phantom test/compliance/VMTest.php'
# OK (1 test)

• Forward profile round-trip still works:

./script/docker-exec.sh -- bash -lc 'export PHP_COMPILER_PROFILE=8.4
source script/php-env.sh && vendor/bin/phpunit --filter brotli_roundtrip test/compliance/VMTest.php'
# OK (1 test)

• Unit: BrotliExtensionPolicyTest, CompilerVersionGateTest::testSupportsBrotli*
• ./script/ci-fast.sh (inventory + spine footnotes regen)

Closes #17563

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