Generate release notes from CHANGELOG, and stop failing on manual runs

[keithlostracco]

Two fixes to the release workflow, both needed before tagging v0.2.0.

Releases were published with an empty description

The Create Release step passed no body, no body_path, and no
generate_release_notes — so nothing populated the release description. Every
set of notes so far was written by hand afterwards (the stored v0.1.2 body has
CRLF line endings, i.e. typed into the web UI). The tags are lightweight, so
there was no tag message to fall back on either.

The body now comes from the CHANGELOG.md entry matching the tag, with
GitHub's generated "What's Changed" appended below it. Notes get written once,
in the PR that made the change, and reviewed there.

Tagging a version with no changelog entry now fails the release instead of
quietly publishing a blank description.

Manual runs always failed

workflow_dispatch is a listed trigger, but a release cannot be published from
a branch. A manual run resolved github.ref_name to main, producing
anim-main-src.zip and name: Release main, then died with
⚠️ GitHub Releases requires a tag. The three build jobs passed; only the
publish step failed.

The release job is now gated on startsWith(github.ref, 'refs/tags/'), so a
manual run skips publishing and acts as a dry run of the cross-platform build
and test gate — which is the only useful thing it could do from a branch.

Verification

The extraction step was run verbatim, parsed straight out of the YAML, against:

• 0.2.0 — produces the full section, no leading or trailing blank lines
• 0.1.2 — stops at the next version heading, no bleed from 0.1.1
• 0.1.0 — the last section; stops before the link-reference block and the
  trailing HTML comment
• 9.9.9 — no such entry, exits 1 with a GitHub error annotation