[Mimic890]

The post cover becomes a full-width banner, right under the top bar and above the title.

• layout.html has a new empty banner block between the top bar and <main>; post.html fills it with the cover. The banner sits outside the 44rem text column, so it spans the page with no 100vw tricks and no horizontal scroll.
• CSS: height: min(30vh, 320px), min(25vh, 200px) up to 30rem; object-fit: cover, centered; no rounded corners; --surface background while loading. width/height attributes are kept. Because the CSS height is fixed, the box size never depends on the image.
• Title, dates and tags stay in the text column. Posts without a cover are unchanged.
• Test: the cover is between the top bar and <main>, and a post without a cover has no banner.
• SPEC (EN + RU): Post page section and Decision log. README (EN + RU): one line on the crop under cover.

Checked with screenshots of a 1600×900 test cover at 1280×900, 1920×1080 and 390×844, light and dark, plus a post without a cover. Heights were 270, 320 and 200 px; the crop was centered; scrollWidth matched the viewport. vet, staticcheck, govulncheck and go test -race pass.