Derive srcset and sizes from layout intent

[romain-pm]

Part of #734. The highest-leverage DevEx item: stop asking call sites to hand-write widths and sizes.

Today a module author must know srcset/sizes semantics, resource-vs-CSS pixels, and DPR to write widths={[300,600]} sizes="300px" — and get it right at every call site (13 in luxe, split into two undocumented conventions, one file explaining its reasoning; a missing sizes silently means 100vw and over-download).

Borrow Astro's layout: declare the intent, derive the rest.

layout	candidates	derived sizes
fixed (needs width)	width, 2×width	<width>px
constrained (needs width)	breakpoints ≤ width, width, 2×width	(min-width: <width>px) <width>px, 100vw
full-width	configured breakpoints	100vw

• layout on getImageProps and the component; default constrained.
• Configurable breakpoint set with a sane default; candidates always clamped to the intrinsic width.
• widths + sizes remain as the explicit escape hatch, not the primary API.

[romain-pm]

Implemented in #746, and the review turned up a documentation gap worth recording here: the API reads as if it second-guesses the width you just gave it, until you know that the slot and the files are two different widths. The slot is what you declare, in CSS pixels; the ladder entries are file widths in image pixels, and one slot legitimately maps to several files because only the browser knows the viewport and the pixel density at load time.

The guide now says which layouts consult the ladder — constrained below its maximum, full-width always, fixed never (its width and that width doubled cover it) — and gives the reason for each limit rather than just the number: candidates stop at 2× the slot because a 3× file roughly doubles the bytes for a difference few people can see, and start at 320 because below that a device scales the 320 file down.