feat: support QIR 2.0 conversion

[qartik]

Summary

Support QIR 2.0 conversion and fold in the LLVM 21 CI, wheel, and release updates needed for this work to validate cleanly.

Closes #3

Included

• QIR 2.0 conversion pipeline and verifier updates
• conversion safety fixes and fixture and snapshot updates
• LLVM 21 test, lint, wheel, and release workflow updates
• Windows-safe Rust and Python smoke examples
• workflow cancellation for superseded PR runs

Limitations

• Windows example smoke runs validate host-target code generation (native, opt_level=0) rather than the cross-target aarch64 optimized path still exercised on Linux and macOS.
• Rationale: the LLVM 21 Windows CI setup has been reliable for host-target smoke coverage, but cross-target aarch64 optimized example compilation has been flaky there. Keeping Windows on the host-target path preserves meaningful API and codegen coverage without blocking the PR on a Windows-specific backend issue.

[Copilot]

Pull request overview

This PR upgrades the QIR-QIS compiler from LLVM 14 to LLVM 21 and adds support for QIR 2.0 (opaque pointer) conversion. The changes span the entire compilation pipeline: parsing, decomposition, conversion, optimization, verification, and testing. Significant effort is spent on Windows compatibility workarounds where LLVM 21 APIs behave differently.

Changes:

• Upgrade from LLVM 14 (llvm-sys 140, inkwell llvm14-0) to LLVM 21 (llvm-sys 211, inkwell llvm21-1), migrating all typed pointer APIs (%Qubit*, i8*, i64*) to opaque pointer (ptr) APIs and updating build_gep/build_load signatures to include explicit types.
• Add QIR 2.0 support: new test fixtures (qir2_base.ll, qir2_adaptive.ll), updated module flag validation to accept qir_major_version 1 or 2, new parse_gep logic to resolve unnamed globals via LLVM C APIs, and direct-to-module decomposition building (replacing module linking + inlining).
• Windows-specific safety workarounds: custom llvm_verify module, #[cfg(windows)] guards for target initialization, CPU feature detection, metadata validation, and test snapshot skipping; plus build system updates (pyproject.toml, Cargo.toml) for LLVM 21 on all platforms.

Reviewed changes

Copilot reviewed 26 out of 27 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
Cargo.toml / Cargo.lock	Upgrade inkwell to llvm21-1 and llvm-sys to 211.0.0
src/lib.rs	Update entry processing to pass array types, Windows cfg guards, QIR 2.0 major version acceptance, trace logging, attribute cleanup changes
src/convert.rs	Migrate to opaque pointer LLVM APIs, new get_required_num_qubits_strict, parse_gep rewrite with LLVM C API fallback, prune_unused_ir_qis_helpers, sanitize_label_for_global_name, test helper changes
src/decompose.rs	Build decompositions directly into module (no linking), add per-step verification, use get_or_create_decomposition_function, change linkage from LinkOnceODR to External for native gates
src/opt.rs	Early return for O0, Windows cfg guards for CPU features and native target init, trace logging
src/llvm_verify.rs	New module for platform-safe LLVM module verification via C API
pyproject.toml	Update build config for LLVM 21 on Linux/macOS/Windows
README.md / qtm-qir-reference.md	Document LLVM 21 requirement and QIR 1.0/2.0 pointer forms
tests/data/qir2_base.ll / tests/data/qir2_adaptive.ll	New QIR 2.0 test fixtures
tests/snaps/*.snap	Updated snapshots for opaque pointer output, new global naming, and reordered declarations

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