Replace LIB_SWIFT_PATH with bundled SwiftCorelibs headers

[Kyle-Ye]

Summary

• Remove libSwiftPath and __COREFOUNDATION_FORSWIFTFOUNDATIONONLY__ from Package.swift, replacing them with bundled system SDK headers under Sources/SwiftCorelibs
• Rename SwiftCorelibsCoreFoundation module to SwiftCorelibs with headers exposed via -isystem include path instead of a module dependency (avoids clang explicit module issues)
• Add dispatch and os headers from swift-corelibs-libdispatch for non-Darwin platforms
• Extract SwiftBridging into its own standalone target
• Support LIB_SWIFT_PATH env variable as an alternative to override bundled headers
• Update README and add per-target READMEs

Test plan

• macOS build passes
• Ubuntu build passes
• Verify LIB_SWIFT_PATH override works on Linux

[augmentcode]

🤖 Augment PR Summary

Summary: This PR switches non-Darwin builds from relying on an external Swift toolchain header path to using bundled SwiftCorelibs headers checked into the repository.

Changes:

• Remove the computed libSwiftPath logic and stop injecting -I include paths for toolchain headers.
• Add a bundled Sources/SwiftCorelibs/include header set (CoreFoundation + dispatch + os) and include it via -isystem on non-Darwin platforms.
• Extract SwiftBridging into its own include target and add it to the shared system include paths.
• Drop the SwiftCorelibsCoreFoundation target/module dependency and rely on headers via include paths instead (avoids explicit module issues).
• Rename the OpenAttributeGraphCxx umbrella header used by the module map.
• Update top-level and per-target READMEs to reflect the new layout and override mechanism.

Technical Notes:OPENATTRIBUTEGRAPH_LIB_SWIFT_PATH can still be used to override the bundled headers when a system toolchain/SDK layout is preferred.

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[augmentcode]

Review completed. 2 suggestions posted.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.12%. Comparing base (acae877) to head (9f09011).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #205 +/- ##
=======================================
Coverage 10.12% 10.12% =======================================
Files 28 28 Lines 820 820 =======================================
Hits 83 83 Misses 737 737 

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