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

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

Closes #8

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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 54c8480 is complete: all 28 checks pass, including Python 3.15 prerelease on Linux, macOS, and Windows. The pull request is clean and ready to merge. The current automation session cannot perform the merge because its integration token receives HTTP 403.