Hardening candidates: per-language demo GIFs on public surfaces

[jeiel85]

Follow-ups surfaced while adding the per-language tablet demo GIFs in #257. None block that PR.

• Landing GIFs ignore prefers-reduced-motion. Six docs/index*.html story-01 figures now animate unconditionally; a GIF cannot be paused by CSS. Swapping to <picture> with a still fallback, or to a muted looping <video> that respects the media query, would let motion-sensitive visitors opt out. Scope: the six landing pages (READMEs are out of reach — GitHub renders them as-is).
• ~930 KB per landing visit on mobile data. The GIF is loading="lazy", so it does not hit LCP, but a phone visitor scrolling to story 01 still pulls ~930 KB. An animated WebP (roughly a third the size) or a poster-plus-<video> swap would cut that. Scope: docs/assets/, the six landing pages.
• Nothing verifies that a language page points at its own GIF. README.ko.md referencing markleaf-tablet-en.gif would render fine and ship silently — the same class of gap that verify-landing-versions.ps1 closed for version strings. A check that every index.<lang>.html / README.<lang>.md references markleaf-tablet-<lang>.gif would catch it. Scope: scripts/, wired into the landing-check workflow.
• markleaf-preview-720/1280.webp are now unreferenced. Kept on purpose (the GitHub Pages URLs may be linked from outside the repo), but nothing records that decision, so a future cleanup will either delete them blind or leave them forever. Scope: decide and note it, or drop them.
• Demo assets have no version trail. The GIFs were recorded on v2.30.0; when the tablet UI changes, nothing flags that the clips are stale — unlike the landing screenshots, which verify-landing-versions.ps1 pins to a figcaption version. Scope: record the recorded-on version next to the assets, or extend the verify script.

[jeiel85]

Audited against the tree at 9969025 (v2.31.0) as part of a sweep over every open hardening tracker. All five items still stand.

• prefers-reduced-motion — landing.css has a @media (prefers-reduced-motion: reduce) block, but the story-01 figures are still plain <img src="assets/markleaf-tablet-<lang>.gif"> on all six pages, and CSS cannot pause a GIF. No <picture> or <video> swap.
• Payload — measured on disk: 941–965 KB per GIF (ko 944 KB, en 951 KB, de 957 KB, fr 954 KB, ja 941 KB, es 965 KB). Still loading="lazy", still ~930 KB+ for a visitor who scrolls to story 01.
• Per-language GIF assertion — scripts/ contains no reference to gif at all; verify-landing-versions.ps1 checks version strings only. A page pointing at another language's clip would still ship silently.
• markleaf-preview-720/1280.webp — confirmed unreferenced (no hits across *.html, *.md, *.css, *.ps1, *.mjs). Both files are still in docs/assets/, and the decision to keep them is still recorded nowhere but this issue.
• Version trail — nothing beside the assets records that they were recorded on v2.30.0.