chore(build): build wheels via soldr PEP 517 backend instead of raw maturin

[zackees]

Two-line [build-system] swap to soldr's PEP 517 backend (requires = ["soldr>=0.7.98"] / build-backend = "soldr"). The [tool.maturin] config is honored byte-identically — soldr's backend drives a pinned maturin with a rustup-pinned toolchain (immune to PATH-shadowing GNU installs), managed cmake/ninja for cmake-based *-sys deps, and rustc caching. Docs: https://github.com/zackees/soldr#use-soldr-as-your-pep-517-build-backend-instead-of-maturin

Part of the org-wide migration (ref zackees/soldr#1264). CI workflows that invoke the maturin CLI directly are intentionally untouched — maturin-as-CLI ignores [build-system].

Validated on a deliberately toolchain-poisoned Windows box: fully-isolated uv pip install . (backend = soldr 0.7.98 from PyPI, manylinux_2_17/win wheels) builds and installs cleanly.

🤖 Generated with Claude Code

[zackees]

CI failure classification — all remaining failures are pre-existing on main

Compared PR run 28590496468 (head 1996747) against main's latest CI run 24317786589. The failing-job sets are identical (8 jobs), and every failure has the same root-cause signature on both runs:

Lane	Failure	Root cause	Main baseline
ubuntu x64 --zig Build	maturin failed … Failed to find zig / cannot find binary path	zig is not installed on the runner; the --zig maturin CLI lanes never worked	identical error, job 70998432339
ubuntu arm64 --zig Build	same	same	identical, job 70998432348
ubuntu x64 --zig Lint	ruff format --check → Would reformat: ci/env.py	unformatted file on main	identical, job 70998432347
ubuntu arm64 --zig Lint	same	same	identical, job 70998432356
macos-15 aarch64 Lint	same ci/env.py	same	identical, job 70998432377
windows x64 Lint	same ci\env.py (build + ruff check pass first)	same	identical, job 70998432330
macos-14 x86_64 Build	uv run fails: built x86_64 wheel not compatible with the current Python 3.11 on macOS aarch64	macos-14 runner is arm64; uv run tries to install the cross-arch wheel into the host venv	identical, job 70998432383
windows aarch64 Build	build_editable fails: host python reports a platform 'win-amd64' … Rust target is 'aarch64' … Couldn't find any python interpreters	same cross-arch editable-install limitation (main hits it via maturin.build_editable, this PR via soldr.build_editable → delegated maturin, same terminal error)	identical, job 70998432333

Evidence the soldr backend works

• All Test jobs pass (ubuntu x64/arm64, macos-15 arm64, windows x64) — the package builds and imports through build-backend = "soldr".
• Build passes on macos-15 aarch64 and windows-2025 x86_64.
• In the windows x64 Lint lane the backend log shows Built my-project … Installed 1 package before the unrelated ruff-format failure.
• In the windows arm64 lane, soldr.build_editable fetched maturin v1.14.1, verified the ninja syslib bundle, and reached the exact same maturin cross-interpreter error main reaches — no soldr-specific regression.

No failures are caused by the backend swap; merging over the documented pre-existing failures.