Update OpenAttributeGraph integration and add Compute backend support

[Kyle-Ye]

Summary

• Align AG environment variable naming with OpenAttributeGraph's OPENATTRIBUTESHIMS_ATTRIBUTEGRAPH / OPENATTRIBUTESHIMS_COMPUTE convention (prefixed with OPENSWIFTUI_)
• Add Compute module backend support as an alternative to AttributeGraph
• Remove swiftToolchain related build logic (SWIFT_TOOLCHAIN_PATH, SWIFT_TOOLCHAIN_VERSION, SWIFT_TOOLCHAIN_SUPPORTED, LIB_SWIFT_PATH) and associated unsafe flags
• Propagate OPENSWIFTUI_ATTRIBUTEGRAPH / OPENSWIFTUI_COMPUTE defines to C and C++ settings (previously Swift-only)
• Update all CI workflows to use the new env var names
• Add Example README with AG backend configuration guide

Test plan

• Verify macOS CI passes with OPENSWIFTUI_OPENATTRIBUTESHIMS_ATTRIBUTEGRAPH=1
• Verify Ubuntu CI passes without AG/Compute enabled
• Verify Compute backend builds with OPENSWIFTUI_OPENATTRIBUTESHIMS_COMPUTE=1

[augmentcode]

🤖 Augment PR Summary

Summary: This PR updates OpenSwiftUI’s OpenAttributeGraph integration by renaming backend-selection environment variables and introducing an opt-in Compute backend path.

Changes:

• Renamed CI/workflow env vars to `OPENSWIFTUI_OPENATTRIBUTESHIMS_ATTRIBUTEGRAPH` (and added docs for `..._COMPUTE`)
• Added Compute backend selection in Package.swift and propagated OPENSWIFTUI_ATTRIBUTEGRAPH/OPENSWIFTUI_COMPUTE defines beyond Swift settings
• Removed Swift toolchain path/version build logic and related unsafe flags from the manifest
• Updated tests to gate/disable specific cases based on attributeGraphVendor (e.g., skipping known-OAG gaps)
• Updated Example project docs and C interpose header selection for multiple backend headers
• Bumped the pinned OpenAttributeGraph dependency revision

Technical Notes: Backend selection is now controlled via OpenAttributeGraphShims-style env flags, and CI is adjusted accordingly.

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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 27.66%. Comparing base (41578a7) to head (637f2f8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #800      +/-   ##
==========================================
+ Coverage   27.58%   27.66%   +0.08%     
==========================================
  Files         657      652       -5     
  Lines       41220    40756     -464     
==========================================
- Hits        11370    11276      -94     
+ Misses      29850    29480     -370     

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