Add code of conduct, issue forms and PR template; test on Python 3.15

[devdavidejesus]

• CODE_OF_CONDUCT.md: Contributor Covenant 3.0, with the reporting contact filled in.
• Issue forms for bug reports and feature requests. Blank issues are off; security reports are pointed to private reporting.
• A pull request template with the checklist from CONTRIBUTING.md.
• README: the roadmap paragraph names the open issues and what stays out of scope. CONTRIBUTING links the code of conduct and the good first issues.
• Python 3.15 (released 2026-10-09) added to the Linux test matrix and the classifiers.

No change to the package code.