docs(plan-2d): extend with title block + post-2C-landing alignment

Plan 2D (q2-preview body container) extended to also cover the document
title block from `template.rs:211-240` (title / subtitle / author / date
/ abstract chrome inside `<header id="title-block-header">`). Phase 7
adds `PreviewTitleBlock`, registered under the synthetic key
`__title_block__` so user TSX can override it through the existing
2C merge site, with `extractMetaStringList` added to `utils/meta.ts`
for YAML list-form authors.

Phase 6 also re-implements the Rust `title-block` transform's
minimal-mode branch on the React side: the transform is excluded from
q2-preview's pipeline (`pipeline.rs:1052`), so without React-side
synthesis q2-preview's minimal mode silently dropped the title.

Aligned to the post-2C-landing files: PreviewDocument prop shape stays
`{ ast, setAst, onNavigateToDocument }`; registry access uses the
`useContext(RegistryContext).registry` pattern from `dispatchers.tsx:39`;
`registry.test.ts` extended to pin the new synthetic key. Multi-author
rendering matches Rust's broken-but-consistent behavior (single block,
empty-string concatenation) — no deliberate divergences.

Author: gordonwoodhull