Allow prerelease fallback at final lower bounds

[matteius]

Summary

• allow prerelease fallback when a candidate is the prerelease of a satisfying final lower bound
• keep PackageFinder and resolvelib requirement checks consistent while preserving exact pins, exclusions, matching final releases, and only-final policy
• update the persistent pip vendoring patch, regression coverage, and Towncrier fragment

Validation

• 801 unit tests passed, 9 skipped under the workspace interpreter
• the remaining 11 unit tests passed in an isolated environment with their optional dependencies
• focused dependency tests: 86 passed
• live end-to-end install and lock selected odin==2.11rc3 for odin~=2.11
• Ruff, Towncrier draft rendering, and git diff --check passed
• vendoring patches reproduced all affected patched-pip files exactly

Closes #6701