CI: build both bindings majors from main - #3175

Summary

Build CUDA 12 and CUDA 13 bindings from their source roots on main, replacing branch-sourced maintenance artifacts.

• For ordinary PRs, reuse unaffected artifacts only from a successful push CI run on the PR base branch at the exact merge-base commit, with the complete required artifact set; otherwise build and test everything. Bindings release tags deliberately select only their matching bindings/metapackage line, without a reusable baseline.
• For a bindings-only change, rebuild its bindings and matching metapackage, reuse the unchanged merged Core wheel, and retain Core tests against the affected bindings major. Core source changes rebuild both ABI variants.
• Build both bindings lines and matching metapackages when required; route artifact downloads and source-distribution checks through the selected root.
• Write each line's build paths and artifact names through the Python bindings_config write-build-env command, with a small shell wrapper for platform setup.
• Keep concise component/major names in the PR checks view, including meta12 and meta13. The workplan summary exposes its JSON, including exact baseline identity and build/test decisions.
• Test meaningful outcomes: selected coverage, correct release artifacts, complete merged Core wheels, and SCM milestones. Remove tests tied to Markdown wording, workflow text, or the exact shape of gh commands.

Version Bootstrap and Platform Boundary

Copying maintenance sources does not make their later branch tags ancestors of main. Package versions still follow standard setuptools-scm progression from the selected line's reachable tags. When suitable history is absent, bindings_scm_version.py creates a local preview tag at a validated ancestor in a disposable build checkout. Existing suitable tags take precedence; the helper never pushes or moves tags.

A plain maintenance checkout is not guaranteed to derive the intended next patch version before an appropriate milestone is reachable. The local preview instructions use an independent disposable clone because linked worktrees share tags. A retained milestone on validated main history, followed by removal of temporary bootstrap calls, remains a separate follow-up.

Windows ARM64 builds omit CUDA 12 because that toolkit has no Windows ARM64 build. Linux and Windows x86-64 retain both registered majors. The next layer supplies the matching test-workflow interfaces.

Stack / Review Boundary

Previous: #3174. Next: #3176. The stack overview lists all six open PRs and the intended workflow.

#3174 through #3176 are intended to land together, with separate review scopes and cumulative CI at #3176; the first two are incomplete intermediate states. Release routing (#3177), maintenance policy (#3178), and the runtime fixes (#3182) follow that group. The complete proposed source tree is at #3182.

Baseline: main 2e6aa4e9ef89.

Suggested Review Order

1. ci/tools/compute_ci_plan.py - build/test selection and exact-baseline reuse.
2. ci/tools/env-vars and bindings_config.py - the small shell wrapper and Python build-environment writer.
3. .github/workflows/build-wheel.yml - builds, baseline downloads, and Core wheel merging.
4. ci/tools/bindings_scm_version.py and the Linux / Windows sdist workflows - temporary local version bootstrap and source-package checks.
5. ci/tools/download-wheels - keep unrelated toolkit lines and test artifacts out of release staging.

Validation

At the exact published heads, 328 CI-tool tests pass at #3176 and 341 at #3182. Full local pre-commit run --all-files --show-diff-on-failure passes at both, including actionlint and Lychee.

Planner regressions cover bindings-only Core reuse, affected-major testing, and full rebuilds when the exact baseline is unavailable. Local Git tests cover SCM bootstrap/progression and preservation of existing tags. A direct comparison matched all 26 build-environment values on Linux x86_64 and aarch64.

This intermediate layer is not independently actionlint-clean: callers pass bindings-config, while reusable test workflows still require build-ctk-ver. #3176 updates those interfaces. Hosted integration: full CI for the combined #3174–#3176 state.

Checklist

• Focused tests protect selection, packaging, and release outcomes that could fail silently.
• This description identifies the layer's scope and validation boundary.