chore: adopt ecosystem qualification

[stefan-jansen]

Summary

• add standardized issue forms, pull request intake, and security reporting guidance
• add the shared Python and operating-system qualification workflow
• make release publication depend on shared qualification
• align package metadata and documentation configuration with ecosystem policy

Verification

• 408 tests, strict documentation build, package build, and pre-commit passed.
• workflow YAML parsed successfully

Closes #35

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[stefan-jansen]

The shared runner and repository-specific Python 3.15 dependency fixes are now on this branch. Final qualification is tracked in ml4t/ecosystem#2 and remains blocked by the accepted upstream Polars datetime failure (pola-rs/polars#28347, fix PR #28750). This PR must remain open until its current stable matrix and all three Python 3.15 prerelease jobs pass.

[stefan-jansen]

Qualification at 3a13150 is complete: every required stable and Python 3.15 core job passes on Linux, macOS, and Windows. The optional CUDA job is correctly skipped on pull requests. The native workflow concluded successfully; GitHub is still displaying its already-successful Build Candidate job as pending in the pull-request check rollup.