Add 'finalize' bump

[CSSFrancis]

The Prepare Release workflow had no way to ship a beta's base as stable, and minor/bugfix/major bumps ignored the bN suffix — so from 0.3.0b2, choosing 'minor' (the intuitive "release it" choice) produced 0.4.0, skipping 0.3.0 entirely.

• Add a 'finalize' bump that drops the bN suffix and releases the current beta's base as stable (0.3.0b2 -> 0.3.0).
• Refuse minor/bugfix/major when the current version is a beta, with a message pointing to 'finalize' (those bumps would skip the in-progress version).
• Error if 'finalize' is run on a non-beta version.
• Derive is_beta for the switcher/redirect steps from the COMPUTED version (ends in bN?) rather than the raw beta checkbox, so finalize is always labelled stable and a mismatched checkbox can't mislabel the docs switcher.

Verified the version math across finalize / guarded / normal paths.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (b4f7953) to head (29f072a).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #36   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files          39       39           
  Lines        3902     3902           
=======================================
  Hits         3519     3519           
  Misses        383      383           

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