[cristim]

Summary

Use Clang's trailing-zero builtin when scanning a nonempty free-page map. This preserves the std::countr_zero result for the nonzero unsigned long long returned by to_ullong() and builds with older libc++ headers that lack the C++20 bit operation.

Why

The target-aware arm64 macOS OpenSwiftUI dependency build compiles OpenAttributeGraph against Darling's current libc++ and failed in table.cpp because std::countr_zero was unavailable.

Validation

• Rebuilt the OpenSwiftUI target with local OpenAttributeGraph sources and Darling's libc++ headers. table.cpp now compiles and the build advances through the C++ sources to Swift errors in Graph.swift (NSDictionary bridging and optional string handling).
• The loop calls the builtin only while free_pages_map.any() is true, so its zero input case is unreachable.

Contributing

Reproduce with a C++20 compile of table.cpp against the older libc++ in Darling and the local Swift runtime headers selected through OpenAttributeGraph PR #242. AI-assisted changes should check the nonzero precondition and verify allocation scans still choose the least significant free bit.