Add boxed-section layout primitive (counterpart to full-bleed/breakout)

[jackgranatowski]

Idea

A named "boxed" section layout primitive — content inset within visible edges (border/shadow/rounded corners framing the section) — as the logical counterpart to the existing .sf-full-bleed / .sf-breakout primitives in core/layout.css.

Why

We already have the "break out of the container" end of the spectrum; we don't have a named pattern for the opposite: a section deliberately framed/inset rather than edge-to-edge. Right now a user has to hand-roll this with raw border/shadow/radius tokens instead of reaching for one documented primitive.

Scope

• Design as a macro (.sf-boxed or similar) built from existing tokens (--sf-radius-*, --sf-border-*, --sf-shadow-*, --sf-gutter) rather than introducing new token groups.
• Should compose with the existing .sf-content-grid/.sf-container system, not duplicate it.
• Update docs/layout.md and docs/llm-guide.md if shipped as a PUBLIC macro.

[jackgranatowski]

Reopening to correct the close reason — this had auto-closed as "completed" when #516 merged, but the PR's final shipped state doesn't actually contain this.

A .sf-boxed section macro was built as part of that PR, then dropped before merge after review: it was judged too thin a wrapper over existing primitives (.sf-section + border/box-shadow/radius tokens) to justify a dedicated class. Re-closing as not planned rather than completed.

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