feat(release): support X.Y.Z-beta-N pre-release tags

[dustenhubbard]

Adopt X.Y.Z-beta-N (e.g. v1.21.0-beta-1) as the fork's pre-release naming, beta-only going forward, alongside the existing PEP 440 (rcN/bN) and dashed-dot (-beta.N) forms.

Changes

• prune_prereleases.py — match a hyphen or dot before the number (-beta-N as well as -beta.N), so a published stable prunes its -beta-N pre-releases. One-char regex change ((?:\.\d+)? → (?:[-.]\d+)?).
• first_launch.py: parse_changelog_section — match a changelog/WHATS_NEW header by parsed PEP 440 version, so a [1.21.0-beta-1] header (setuptools-scm bakes the tag into the app as 1.21.0b1) matches a 1.21.0b1 runtime; string fallback kept for non-version headers ([Unreleased]).

Notes

• The live What's-new dialog was already correct — whats_new_content already matches by parsed version. This only hardens the currently-unused parse_changelog_section helper so it can't become a footgun.
• No CI change needed: Classify release already treats non-vX.Y.Z tags as pre-release, and the release-notes step + updater asset parsing already handle the verbose tag string.
• Tests cover the -beta-N form in both the prune selector and the changelog matcher.