[bmscomp]

Why

The Kates YouTube channel needs a banner, and the drafts lived only as untracked files in website/assets. That folder is the deployed site, and the site never links to them. Two of the drafts also spelled the domain katesplatform.io; the site's canonical domain is katesplatforme.io. This PR keeps all seven proposals in one documented place, so one can be picked and uploaded.

What changes

• New folder docs/assets/youtube-banners/: seven 2560 × 1440 banners, each an SVG with its PNG export beside it. It sits next to the other brand assets in docs/assets: the logo, the wordmarks and the social preview.
• Wordmark: every banner writes "Kates" the way the book cover does. The cover's EB Garamond title (docs/book/assets/cover.svg, #title) is copied as outlines, with the K in blue and "ates" in white. A script check confirmed the paths are identical to the cover's. Brand Streams is the exception: it is the one light banner, so its "ates" uses the cover's navy.
• Five new proposals: Brand Streams, Broker Down, Append-only Log, Engineering Blueprint and CI Gate. An independent review checked each against YouTube's safe area (1546 × 423), legibility at phone size, the facts it states, and the logo. They scored 8 to 8.5 out of 10 with no blockers. Each keeps every word at 34 px or more.
• Two older drafts: Terminal and Telemetry. They now use the cover wordmark and the correct domain.
• README.md: scores each proposal, names its main risk, shows a preview of each, and explains what each device crops and how to render and upload a banner.

Deliberately not done:

• The Terminal and Telemetry drafts still print throughput, P99, data-loss and conformity figures as if measured, and most of their text is too small to read on a phone. The README says so; they were not reviewed or reworked.
• website/assets/kates-youtube-avatar.* is untracked on my machine but is not part of this PR.

Tests

The changed paths trigger only the Docs workflow. I ran its jobs locally:

• scripts/gen-chart-table.sh --check: OK, no chart-table markers in README.md.
• scripts/gen-version-matrix.sh --check: OK.
• scripts/check-book-style.sh: OK.
• scripts/check-book-contrast.py: OK, 170 colour pairs.
• scripts/check-mermaid.sh --quiet: OK, 161 diagrams parse.
• Markdown link check: lychee isn't installed here, so a script checked the README's 15 relative links and image sources; none are missing. CI's lychee job runs the real check.