fix(validate): report requirements outside delta sections (#1804)

* fix(validate): report requirements outside delta sections

* docs(test): document orphaned-requirement test helpers

* test(parser): pin orphan reporting to the reader's section rule

Cover a header the reader does not match exactly (`## ADDED  Requirements`),
which must be reported, and a repeated `## ADDED Requirements` header with a
non-delta section between the copies, which must not. Add a patch changeset
matching the other parser fixes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Co-authored-by: Clay Good <hi@claygood.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>