TRIOS_PHD_NO_IMAGE_TRAIN: anchor hero panels via Needspace, ban hard clearpage

[gHashTag]

Problem

The TRIOS PhD monograph renderer can lay out hero panels as a gallery / image train when section breaks and figure placements interact badly. Two failure modes seen in recent builds:

1. A section heading sits orphaned at the bottom of a page with its hero image floating to the next page.
2. Hero figures from neighbouring sections collapse onto a single page, producing a multi-image "image train" without semantic context.

A previous attempt to fix this by inserting a hard \clearpage before every section produced short title-only pages and was rejected by manual QA.

Rule (non-negotiable)

TRIOS_PHD_NO_IMAGE_TRAIN: each hero panel must travel with the nearest substantive heading and its body text. Use a SOFT keep-together reservation; never a hard \clearpage.

Required implementation

• Add \usepackage{needspace} to docs/phd/main.tex and templates/chapter.template.tex.
• Wrap \section and \chapter to prepend \Needspace*{0.58\textheight} (soft keep-together; the starred form skips the break when already at top of page).
• Wrap \chapterhero so heading + hero figure cannot be split.
• Keep hero images large and centered — this is a placement rule, not a sizing rule.
• Document the rule in docs/pdf-rendering.md (canonical) and link it from AGENTS.md and docs/phd/README.md.

QA baseline to preserve

150 A4 pages; qpdf --check clean; 0 duplicate long paragraphs; 0 duplicate numbered headings; 0 Cyrillic hits in the body; 0 secret / stale / math anomaly hits; 0 very-short non-empty pages; at most 1 image-heavy / low-context page (the title page).