Bootstrap: sync spine for BuiltinInternalDefaultValues (#18356)

[PurHur]

Summary

• Add lib/BuiltinInternalDefaultValues.php to compiler_lib_spine_smoke (landed via Stdlib: ReflectionParameter internal ctor — isOptional false and getDefaultValue() missing (ext/reflection/php_reflection.c) #18356 but missing from spine).
• Regenerate docs/bootstrap-inventory.md so inventory counts stay in sync.

Fixes red check-selfhost-spine-coverage-sync / north-star5-verify-fast step 2 on master.

Refs #18356, #1492

Test plan

php script/check-selfhost-spine-coverage-sync.php
# check-selfhost-spine-coverage-sync: OK (spine covers all 4692 inventory files)

php script/bootstrap-inventory.php --check
# OK 4692/4692

./script/docker-exec.sh -- bash -lc 'make bootstrap-selfhost-link'
# bootstrap-selfhost-link: OK /compiler/build/selfhost

./script/docker-exec.sh -- bash -lc 'make north-star5-verify-fast'
# north-star5-verify: OK (fast — Spine: 4692/4692, step 4f2 VM driver probe OK)

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