Release 10.0.0: date the changelogs and tag v10.0.0

Change

The tag-time steps in CONTRIBUTING.md ("Branches and releases" and "Changelog"):

Set all five ## [10.0.0] headings to the tag date, 2026-10-19, in the commit that gets tagged.
Tag v10.0.0 on that commit. The release workflow verifies and packs the four packages, waits for the nuget environment's approval, publishes, and creates the GitHub release from the root changelog section.
Cut release/10.x from the tag.

The retarget of main to .NET 11, with the version bump and the baseline and SECURITY.md moves, follows in #103 (#280).

v10.0.0 is tagged, the four packages are on NuGet at 10.0.0, the GitHub release exists, and release/10.x exists.

chore: date the 10.0.0 changelogs for the v10.0.0 tag

Done 2026-10-19.

v10.0.0 is tagged on d47d9a3, the merge of chore: date the 10.0.0 changelogs for the v10.0.0 tag #285.
Release run 39219806271: verify and pack, publish (approved on the nuget environment) and GitHub release all succeeded. nuget.org accepted all four packages and their symbol packages at 10.0.0.
The GitHub release is the latest release.
release/10.x is cut at d47d9a3 and covered by the release/** ruleset.

Next on main: the retarget to .NET 11, with the version, baseline and SECURITY.md moves (#103, #280).