[WIP] Toolchain improvements.

[gjz010]

• Toolchaining with devenv, Nix-based reproducible development environment manager.
• Switching to battery-included build of monodromy.
  • This build of monodromy is carefully built with portability in mind and does not require separated building of lrslib. Works on Linux and Windows.
  • Main issue is license: this may resulting in entire repo GPLv3 rather than Apache 2.0 since lrslib is GPLv2+.
• Added tests for SQiSW decomposition.
• WIP: backport RIIR CanopusMapping implementation.
  • The current canopus.utils._accel extension merely contains a lot of small functions and would in fact result in longer execution time due to Rust-Python FFI boundary.

Summary by Sourcery

Introduce a reproducible Rust+Python development environment and strengthen SQiSW-based two-qubit synthesis validation.

New Features:

• Add Haar-random and structured test coverage for SQiSW-based two-qubit unitary decomposition, including gate-count validation by KAK invariants.

Enhancements:

• Switch monodromy to a prebuilt portable wheel and configure uv to track Rust and Python project files for automatic rebuilds.
• Add temporary diagnostics around KAK decomposition to inspect unitaries and their interaction coefficients.

Build:

• Add Nix-based devenv configuration (including Rust, Python with uv, and treefmt) and associated lock files for reproducible local development.

Tests:

• Introduce comprehensive SQiSW decomposition test suite covering canonical gates, local unitaries, and Haar-random SU(4) distributions.

[sourcery-ai]

Reviewer's Guide

Introduces a Nix/devenv-based reproducible dev environment, switches monodromy to a prebuilt wheel sourced via uv, and adds an extensive SQiSW decomposition test suite along with minor Rust/Python plumbing tweaks to support and debug two-qubit KAK-based logic.

File-Level Changes

Change	Details	Files
Add uv/devenv-based reproducible development environment configuration.	Introduce devenv.nix to configure Rust (stable), Python with uv and venv, Nix LSP, and treefmt with nixfmt/rustfmt. Add devenv.yaml to declare flake-style inputs (rust-overlay, treefmt-nix) for devenv. Check in devenv.lock and uv.lock to pin environment dependencies. Add .envrc scaffold (likely for direnv integration) and update .gitignore if needed for new environment artifacts.	devenv.nix devenv.yaml devenv.lock uv.lock .envrc .gitignore
Switch monodromy dependency to a prebuilt wheel fetched directly by uv and configure Rust rebuild cache keys.	Configure [tool.uv.sources] in pyproject.toml to point monodromy to a GitHub release wheel URL. Add [tool.uv] cache-keys to trigger Rust extension rebuilds when Rust or dependency files change (pyproject, Cargo.toml/lock, src/**/*.rs).	pyproject.toml
Tidy Rust canonical_unitary binding signature formatting for readability.	Reformat canonical_unitary pyfunction declaration to multi-line argument style without changing behavior.	src/lib.rs
Instrument KAK decomposition helper for debugging Weyl coordinates.	Temporarily add prints of the unitary and interaction coefficients inside _kak_from_unitary for inspection.	python/canopus/decomposition/utils.py
Add a comprehensive SQiSW decomposition correctness and distribution test suite.	Define an SQiSWGate wrapper using Qiskit’s with_gate_array/SingletonGate to obtain the √iSWAP matrix. Implement helpers to compare unitaries up to global phase, count SQiSW gates in circuits, and synthesize unitaries from given Weyl coordinates with random local unitaries. Add TestSQiSWDecomposition tests for minimal named-gate cases and fixed random examples ensuring expected SQiSW counts (0–3). Add torture tests for measure-zero SU(2)⊗SU(2) local unitaries ensuring zero SQiSWs, and for Haar-random SU(4) unitaries verifying KAK-based expected SQiSW counts and empirical distribution (~79% 2-SQiSW, rest 3-SQiSW). Include an intentional failing assert in check_equivalence (after the overlap check) to force tests to fail while debugging, ensuring this is revisited before merge.	tests/test_sqisw.py

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