Update minimum Python version to 3.13 and pin numpy to 2.5.2

[flagrama]

Add draft of dependency versioning policy document as well.

This helps resolve multiple issues discovered via real-world Windows and Linux usage discussed in #dev-public-talk as well as creating consistent documentation for bumping dependency versions with a proposed policy written in the document draft.

[fenhl]

The previous policy in CONTRIBUTING.md should be replaced with a reference to the new document.

[flagrama]

Note that I don't mention Linux as being a platform necessary for dependency policies for several reasons:

1. Windows is the actual least-common-denominator development platform
2. Different distros change what needs changed to work so it's hard to have something "consistent"
3. Linux can always be coerced into working from as little as "compile/install an updated package" to as much as "effectively create a second copy of Linux as a development environment" and development contributors will usually know which one is necessary for their platform to get things to work

It does not imply Linux is not a supported version for runtime in any way, nor does it imply Linux is not a supported development platform.