Issues/0432 - Harden release gating: resilient redlines, visible errors, draft pruning, docs

[SteveLLamb]

What

• build.mjs — Edition/release redlines now warn and skip when a reference tag fails to render, instead of failing the whole build. A premature or non-building release tag can no longer wedge every build. (PR-base redline stays strict.)
• build.mjs — Forward page pageerror/console errors to the build log, so render failures show the real cause instead of just Page has errors.
• action.yml — Prune prior draft releases for the same tag before creating a new draft (published releases untouched), so auto-drafts stop piling up. Auto-draft already runs only after a successful build.
• doc/main.html — C.5: publish with Set as the latest release (not pre-release) (Add "select the 'pre-release' Release Label" in C.5 Publishing the draft release #425). Auto-draft annex: releases come from the auto-created draft, not a manual publish on an arbitrary commit.

Why

Surfaced on st2094-50-private: a release published on a never-green commit wedged every subsequent build at the new release-redline step, with an opaque Page has errors. These changes make the redline non-fatal, the failure visible, and draft clutter self-cleaning.

Closes #432 and closes #425

[github-actions]

Review links

Clean
Redline to current draft
Redline to most recent published edition