[burgholzer]

🤖 AI text below 🤖

Description

Compiler Program APIs currently convert MLIR results to bool or std::optional, and callers convert them back. Use LLVM's LogicalResult and FailureOr<T> directly in fallible Program operations and the benchmark generators that produce Programs. Remove the private boolean linearity wrapper and the OpenQASM binding's optional-to-result conversion.

This extracts the independent Program API cleanup from #2545. Ownership, diagnostics, Python APIs, and Python exception categories are preserved. C++ callers must replace boolean or has_value() checks on these results with failed() or succeeded(); the source-tree API example documents this migration.

Two separate commits restore the HOME/.config fallback when XDG_CONFIG_HOME is empty and remove the configuration paragraph about empty environment variables. The registry regression covers both unset and empty XDG values.

Codex implemented the changes, ran local validation, and performed the requested ponytail review. The review found no actionable correctness or simplification issues.

Validation on Linux aarch64 with GCC 13.3, LLVM/MLIR 23.1, and Python 3.14:

• Release build and CTest: 4,006 tests passed, one existing test skipped.
• Rebuilt wheel: all 1,960 Python tests passed.
• Stub generation: passed with no generated API changes.
• Repository lint and whole-file C++ lint across 31 files: passed.
• Executable documentation and local documentation links: passed after clearing stale generated pages from ♻️ Return owned errors at native boundaries #2545.

Windows, macOS, and hosted CI results remain to be checked.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• Any agent that created, edited, or submitted GitHub content was explicitly authorized for that scope, as required by our AI Usage Guidelines.
• Every agent-authored or agent-edited public text body begins with the visible disclosure 🤖 *AI text below* 🤖 (titles are exempt).
• I have disclosed AI assistance in the PR description.
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[codecov]

Codecov Report

❌ Patch coverage is 82.69231% with 27 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mlir/lib/Compiler/Pipeline.cpp	83.6%	17 Missing ⚠️
mlir/bench/Generate.cpp	40.0%	3 Missing ⚠️
mlir/lib/Compiler/Programs.cpp	88.0%	3 Missing ⚠️
mlir/lib/Compiler/ParameterBinding.cpp	77.7%	2 Missing ⚠️
mlir/lib/Compiler/QDMIAdapter.cpp	66.6%	1 Missing ⚠️
src/qdmi/devices/dd/WorkerMain.cpp	83.3%	1 Missing ⚠️

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