ci: add selective sdist build plumbing

[kkraus14]

Important

Stack dependency: PR 2 of 4. Builds on #2464, which has merged. #2466 and #2467 depend on this PR.

Merge order: #2464 (merged) -> #2465 (this PR) -> #2466 -> #2467. All branches live in the kkraus14 fork and target upstream main.

What

• Add default-on package build inputs to both sdist reusable workflows.
• Skip package-specific setup and sdist builds when that package is not selected.
• Hydrate unchanged cuda-pathfinder and cuda-bindings wheels from the current CI run when a selected downstream sdist needs them.
• Integrate selective downloads with ci: constrain internal builds to exact local wheels #2510's exact local-wheel constraints, without PIP_PRE or PIP_FIND_LINKS.
• Preserve the current Linux and Windows compiler, cache, CTK, timeout, and path behavior.

Why

Selective wheel builds are not enough on their own: the sdist smoke jobs also rebuild every package today. This plumbing lets the final orchestration PR apply the same dependency closure to sdist coverage.

Behavior before activation

Every new input defaults to true, and this PR does not change any caller. Sdist coverage remains unchanged until the orchestration PR supplies selective values.

Validation

• Full pre-commit hooks for both workflows
• YAML and GitHub Actions lint
• Review of all build-flag combinations and Linux/Windows artifact dependencies
• Verified the final diff contains only the two sdist workflows on top of merged ci: add selective wheel build plumbing #2464/current main

Related to #299.

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[mdboom]

LGTM

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