test(spec): pin the author-visible message at every string-or-object union site that has one (#17576)

* wip(spec): union author-message pins — first draft

Claude-Session: https://claude.ai/code/session_01MkQhmuuJAVDjmeWNixwDDH
Co-authored-by: Claude <noreply@anthropic.com>

* test(spec): pin the author-visible message at every string-or-object union site that has one

Claude-Session: https://claude.ai/code/session_01MkQhmuuJAVDjmeWNixwDDH
Co-authored-by: Claude <noreply@anthropic.com>

* test(spec): anchor the population reading to the branch point and its re-derivations

Claude-Session: https://claude.ai/code/session_01MkQhmuuJAVDjmeWNixwDDH
Co-authored-by: Claude <noreply@anthropic.com>

* test(spec): add the dropped FormSection.fields site, correct the class-C reason, list the exclusions

Claude-Session: https://claude.ai/code/session_01MkQhmuuJAVDjmeWNixwDDH
Co-authored-by: Claude <noreply@anthropic.com>

* chore: merge origin/main into claude/issue-15423-union-message-pins

* test(spec): make pin 2's selected-branch count real, and retire the retracted class-C sentence

Claude-Session: https://claude.ai/code/session_01MkQhmuuJAVDjmeWNixwDDH
Co-authored-by: Claude <noreply@anthropic.com>

* chore: merge origin/main into claude/issue-15423-union-message-pins

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Co-authored-by: Claude <noreply@anthropic.com>

Author: os-bill