Post image carousel

Render the images bundled with a post as a responsive carousel.

Replace the stub be-best/layouts/_partials/components/carousel.html, which prints a CAROUSEL label and a bare <img> per resource with an empty alt, and which no layout includes. Retire the matching TODO.md line, “Prep work for implementing a carousel again”.

Post image carousel

Enumerate the images of the page in filename order, excluding featured.webp.
Render each slide with the same <picture> markup as be-best/layouts/_markup/render-image.html: webp and jpeg srcsets built from the 320, 640, 960, 1280, 1600 and 1920 widths the resource can supply, a 640px jpeg fallback src, width and height, loading="lazy", and the imgB-<md5>-LQIP background class.
Set alt from the resource filename stem: carousel-03.webp becomes carousel 03.
Add previous and next controls, keyboard arrow-key support, and a slide counter in the form 3 / 9.
Include the partial from be-best/layouts/post/page.html.
A post with no image resource renders no carousel markup; a post with one renders the image with no controls and no counter.

Acceptance Criteria

public/post/garden/spring-2021-try-again/index.html holds nine <picture> elements ordered carousel-01.webp to carousel-09.webp, excludes featured.webp, and shows a counter reading 1 / 9.
content/post/general/building-a-joystick/ yields four slides, arcade-joystick-01.jpeg to arcade-joystick-04.jpeg.
grep -r 'CAROUSEL' public/ prints nothing.
Every carousel <img> has a non-empty alt, width, height and loading="lazy".
pnpm run test passes, including the single h1 heading check and pnpm run test:kanban.