Resolve the markdown-preview revert and README proofreading against this
PR's trajectory inspection work:
- CodeBlock keeps `showHeader` (the PTC code inspector supplies its own
toolbar) on top of master's `contentRef` scrollport wrapper.
- ui-primitives README records both `contentRef` and `showHeader`.
- ui-trajectory README keeps master's proofread wording alongside this
PR's PTC inspection and timing paragraphs.
- Re-record the README.i18n.yaml pairing hashes for the three merged pairs.