feat: support Python 3.15 with verified Amulet compatibility Wheels

[ReallocAll]

Summary

• Extend EndKeep's Python compatibility window from 3.12–3.14 to 3.12–3.15 on Linux and Windows x86-64.
• Keep Amulet-LevelDB's 3.0.7a0 implementation unchanged; build CPython 3.15 Wheels from the exact upstream commit, validate Linux/Windows native DB operations, publish isolated compatibility prerelease, and pin both assets by SHA256.
• Extend the compatibility CI matrix to eight combinations, resolving published Amulet Wheels from a clean environment. The Python 3.15 leg uses the successful Endstone 0.11.14.dev6 Build, run 38045137884, rather than an incompatible older Endstone wheel.
• Update installation and contributor guidance.

Test evidence

• Amulet CPython 3.15 build and smoke tests: Linux + Windows PASS, verified SHA256 manifest, compatibility assets published at release.
• Real BDS lab candidate: in progress. BDS lab checks Endstone cp315 artifact provenance, plugin load, capture, maintenance and deep verification.
• EndKeep compatibility and build CI are running on this PR.

Do not tag or publish an EndKeep plugin release automatically. The compatibility assets remain separately licensed under the Amulet Team License.